diff client/packhand.c client/packhand.c
--- client/packhand.c	Mon Oct 16 10:38:31 2000
+++ client/packhand.c	Tue Oct 17 10:55:42 2000
@@ -235,15 +235,15 @@
   pcity=find_city_by_id(packet->id);
 
   if (pcity && (pcity->owner != packet->owner)) {
-    /* With current server, this shouldn't happen: when city changes
-     * owner it is re-created with a new id.  Unclear what to do here;
-     * try to cope...
-     */
+    /* If we receive full city packet because we own this city, city of
+       old owner should be already removed. If we are just investigating
+       city, we should have already received packet_short_city with new
+       owner. */
     freelog(LOG_ERROR,
-	    "Got existing city id (%d %s) with wrong owner (%d %s, old %d %s)",
-	    packet->id, pcity->name, packet->owner,
-	    get_player(packet->owner)->name, pcity->owner,
-	    get_player(pcity->owner)->name);
+            "Got existing city id (%d %s) with old owner (%d %s, old %d %s)",
+            packet->id, pcity->name, packet->owner,
+            get_player(packet->owner)->name, pcity->owner,
+            get_player(pcity->owner)->name);
     client_remove_city(pcity);
     pcity = 0;
   }
@@ -446,15 +446,6 @@
   pcity=find_city_by_id(packet->id);
 
   if (pcity && (pcity->owner != packet->owner)) {
-    /* With current server, this shouldn't happen: when city changes
-     * owner it is re-created with a new id.  Unclear what to do here;
-     * try to cope...
-     */
-    freelog(LOG_ERROR,
-	    "Got existing city id (%d %s) with wrong owner (%d %s, old %d %s)",
-	    packet->id, pcity->name, packet->owner,
-	    get_player(packet->owner)->name, pcity->owner,
-	    get_player(pcity->owner)->name);
     client_remove_city(pcity);
     pcity = 0;
   }
diff common/city.h common/city.h
--- common/city.h	Mon Sep 25 17:35:40 2000
+++ common/city.h	Mon Oct 16 12:10:18 2000
@@ -152,6 +152,7 @@
 
 struct city {
   int id;
+  int orig_id;
   int owner;
   int x, y;
   char name[MAX_LEN_NAME];
diff server/cityhand.c server/cityhand.c
--- server/cityhand.c	Mon Sep 25 17:35:44 2000
+++ server/cityhand.c	Tue Oct 17 11:11:05 2000
@@ -156,6 +156,7 @@
   pcity=fc_malloc(sizeof(struct city));
 
   pcity->id=get_next_id_number();
+  pcity->orig_id = pcity->id;
   idex_register_city(pcity);
   pcity->owner=pplayer->player_no;
   pcity->x=x;
@@ -283,7 +284,7 @@
 				   struct packet_city_request *preq)
 {
   struct city *pcity;
-  pcity=find_city_by_id(preq->city_id);
+  pcity=find_city_by_orig_id(preq->city_id);
   if(!pcity) 
     return;
   if(!player_owns_city(pplayer, pcity))  
@@ -331,7 +332,7 @@
 {
   struct city *pcity;
 
-  pcity=find_city_by_id(preq->city_id);
+  pcity=find_city_by_orig_id(preq->city_id);
   if(!pcity) 
     return;
   if (!player_owns_city(pplayer, pcity))  return;
@@ -359,7 +360,7 @@
 			     struct packet_city_request *preq)
 {
   struct city *pcity;
-  pcity=find_city_by_id(preq->city_id);
+  pcity=find_city_by_orig_id(preq->city_id);
 
   if (preq->worker_x < 0 || preq->worker_x > 4)
     return;
@@ -439,7 +440,7 @@
 void handle_city_sell(struct player *pplayer, struct packet_city_request *preq)
 {
   struct city *pcity;
-  pcity=find_city_by_id(preq->city_id);
+  pcity=find_city_by_orig_id(preq->city_id);
   if (!pcity || !player_owns_city(pplayer, pcity) 
       || preq->build_id>=game.num_impr_types) 
     return;
@@ -523,7 +524,7 @@
 void handle_city_worklist(struct player *pplayer, struct packet_city_request *preq)
 {
   struct city *pcity;
-  pcity=find_city_by_id(preq->city_id);
+  pcity=find_city_by_orig_id(preq->city_id);
 
   copy_worklist(pcity->worklist, &preq->worklist);
 
@@ -536,7 +537,7 @@
 void handle_city_buy(struct player *pplayer, struct packet_city_request *preq)
 {
   struct city *pcity;
-  pcity=find_city_by_id(preq->city_id);
+  pcity=find_city_by_orig_id(preq->city_id);
   really_handle_city_buy(pplayer, pcity);
 }
 
@@ -546,7 +547,7 @@
 void handle_city_refresh(struct player *pplayer, struct packet_generic_integer *preq)
 {
   struct city *pcity;
-  pcity=find_city_by_id(preq->value);
+  pcity=find_city_by_orig_id(preq->value);
   if (pcity) {
     city_refresh(pcity);
     send_city_info(pplayer, pcity);
@@ -630,7 +631,7 @@
 			struct packet_city_request *preq)
 {
   struct city *pcity;
-  pcity=find_city_by_id(preq->city_id);
+  pcity=find_city_by_orig_id(preq->city_id);
   if (!pcity) {
     freelog(LOG_ERROR, "Pcity null in handle_city_change"
 	                " (%s, id = %d)!", pplayer->name, preq->city_id);
@@ -664,7 +665,7 @@
   char *cp;
   struct city *pcity;
   
-  pcity=find_city_by_id(preq->city_id);
+  pcity=find_city_by_orig_id(preq->city_id);
 
   if(!player_owns_city(pplayer, pcity))
     return;
@@ -685,7 +686,7 @@
 void handle_city_options(struct player *pplayer,
 				struct packet_generic_values *preq)
 {
-  struct city *pcity = find_city_by_id(preq->value1);
+  struct city *pcity = find_city_by_orig_id(preq->value1);
   if (!pcity || pcity->owner != pplayer->player_no) return;
   pcity->city_options = preq->value2;
   /* We don't need to send the full city info, since no other properties
@@ -1014,7 +1015,7 @@
 {
   int i, x, y;
   char *p;
-  packet->id=pcity->id;
+  packet->id=pcity->orig_id;
   packet->owner=pcity->owner;
   packet->x=pcity->x;
   packet->y=pcity->y;
@@ -1028,7 +1029,7 @@
   packet->ppl_scientist=pcity->ppl_scientist;
   packet->ppl_taxman=pcity->ppl_taxman;
   for (i=0;i<4;i++)  {
-    packet->trade[i]=pcity->trade[i];
+    packet->trade[i]=orig_city_id(pcity->trade[i]);
     packet->trade_value[i]=pcity->trade_value[i];
   }
 
@@ -1105,7 +1106,7 @@
     packet->happy=1;
   }
 
-  if ((pcity = map_get_city(x,y)) && pcity->id == pdcity->id &&
+  if ((pcity = map_get_city(x,y)) && pcity->orig_id == pdcity->id &&
       city_got_building(pcity,  B_PALACE))
     packet->capital=1;
   else
@@ -1126,13 +1127,13 @@
   struct dumb_city *pdcity;
   if (!plrtile->city) {
     plrtile->city = fc_malloc(sizeof(struct dumb_city));
-    plrtile->city->id = pcity->id;
+    plrtile->city->id = pcity->orig_id;
   }
   pdcity = plrtile->city;
-  if (pdcity->id != pcity->id) {
+  if (pdcity->id != pcity->orig_id) {
     freelog(LOG_ERROR, "Trying to update old city (wrong ID)"
 	    " at %i,%i for player %s", pcity->x, pcity->y, pplayer->name);
-    pdcity->id = pcity->id;   /* ?? */
+    pdcity->id = pcity->orig_id;   /* ?? */
   }
   sz_strlcpy(pdcity->name, pcity->name);
   pdcity->size = pcity->size;
@@ -1151,7 +1152,7 @@
 
   if (pdcity) {
     pcity = map_get_tile(x,y)->city;
-    if (!pcity || (pcity && pcity->id != pdcity->id)) {
+    if (!pcity || (pcity && pcity->orig_id != pdcity->id)) {
       packet.value=pdcity->id;
       lsend_packet_generic_integer(&pplayer->connections, PACKET_REMOVE_CITY,
 				   &packet);
diff server/citytools.c server/citytools.c
--- server/citytools.c	Sat Sep  2 11:22:06 2000
+++ server/citytools.c	Mon Oct 16 12:12:14 2000
@@ -1064,6 +1064,7 @@
   *pnewcity->worklist = *pcity->worklist;
 
   pnewcity->id=get_next_id_number();
+  pnewcity->orig_id = pcity->orig_id;
   idex_register_city(pnewcity);
 
   for (i = 0; i < game.num_impr_types; i++) {
@@ -1163,3 +1164,43 @@
   }
 }
 
+
+int orig_city_id(int id)
+{
+  if(!id)
+    return 0;
+  return find_city_by_id(id)->orig_id;
+}
+
+
+struct city *find_city_by_orig_id(int id)
+{
+  int i;
+  struct city *pcity;
+  struct genlist_iterator myiter;
+
+  /* Most of the time cities id == orig_id. */
+  pcity = find_city_by_id(id);
+  if(pcity)
+    return pcity;
+
+  for(i=0; i<game.nplayers; i++) {
+    genlist_iterator_init(&myiter, &game.players[i].cities.list, 0);
+    for(; ITERATOR_PTR(myiter); ITERATOR_NEXT(myiter))
+      if(id == ((struct city *)ITERATOR_PTR(myiter))->orig_id)
+        return (struct city *)ITERATOR_PTR(myiter);
+  }
+  return 0;
+}
+
+
+struct city *player_find_city_by_orig_id(struct player *pplayer, int city_id)
+{
+  struct city *pcity = find_city_by_orig_id(city_id);
+  
+  if(pcity && (pcity->owner==pplayer->player_no)) {
+    return pcity;
+  } else {
+    return NULL;
+  }
+}
diff server/citytools.h server/citytools.h
--- server/citytools.h	Tue Jul 25 21:18:35 2000
+++ server/citytools.h	Mon Oct 16 12:10:16 2000
@@ -69,4 +69,9 @@
 
 void adjust_city_free_cost(int *num_free, int *this_cost);
 
+int orig_city_id(int id);
+struct city *find_city_by_orig_id(int id);
+struct city *player_find_city_by_orig_id(struct player *pplayer, int city_id);
+
+
 #endif  /* FC__CITYTOOLS_H */
diff server/diplhand.c server/diplhand.c
--- server/diplhand.c	Fri Sep  8 16:42:36 2000
+++ server/diplhand.c	Mon Oct 16 12:10:18 2000
@@ -279,6 +279,7 @@
   struct Treaty *ptreaty;
   struct player *plr0, *plr1, *pgiver;
   int capability;
+  int real_value;
 
   plr0=&game.players[packet->plrno0];
   plr1=&game.players[packet->plrno1];
@@ -294,10 +295,13 @@
    */
 
   if(packet->clause_type == CLAUSE_CITY){
-    struct city *pcity = find_city_by_id(packet->value);
+    struct city *pcity = find_city_by_orig_id(packet->value);
     if (pcity && !map_get_known_and_seen(pcity->x, pcity->y, plr1->player_no))
       give_citymap_from_player_to_player(pcity, plr0, plr1);
+    real_value = pcity->id;
   }
+  else
+    real_value = packet->value;
 
   capability = 1;
   conn_list_iterate(plr0->connections, pconn) {
@@ -318,7 +322,7 @@
   }
 
   if((ptreaty=find_treaty(plr0, plr1))) {
-    if(add_clause(ptreaty, pgiver, packet->clause_type, packet->value)) {
+    if(add_clause(ptreaty, pgiver, packet->clause_type, real_value)) {
       lsend_packet_diplomacy_info(&plr0->connections, 
 				 PACKET_DIPLOMACY_CREATE_CLAUSE, 
 				 packet);
@@ -427,7 +431,10 @@
 
 	packet.clause_type = pclause->type;
 	packet.plrno_from = pclause->from->player_no;
-	packet.value = pclause->value;
+	if(pclause->type == CLAUSE_CITY)
+          packet.value = orig_city_id(pclause->value);
+        else
+          packet.value = pclause->value;
   
 	send_packet_diplomacy_info(dest, PACKET_DIPLOMACY_CREATE_CLAUSE,
 				   &packet);
diff server/savegame.c server/savegame.c
--- server/savegame.c	Mon Sep 25 17:35:44 2000
+++ server/savegame.c	Mon Oct 16 12:10:18 2000
@@ -707,6 +707,9 @@
     pcity->id=secfile_lookup_int(file, "player%d.c%d.id", plrno, i);
     alloc_id(pcity->id);
     idex_register_city(pcity);
+    pcity->orig_id=
+      secfile_lookup_int_default(file, pcity->id, "player%d.c%d.orig_id",
+                                 plrno, i); 
     pcity->owner=plrno;
     pcity->x=secfile_lookup_int(file, "player%d.c%d.x", plrno, i);
     pcity->y=secfile_lookup_int(file, "player%d.c%d.y", plrno, i);
@@ -1289,6 +1292,7 @@
 
     i++;
     secfile_insert_int(file, pcity->id, "player%d.c%d.id", plrno, i);
+    secfile_insert_int(file, pcity->orig_id, "player%d.c%d.orig_id", plrno, i);
     secfile_insert_int(file, pcity->x, "player%d.c%d.x", plrno, i);
     secfile_insert_int(file, pcity->y, "player%d.c%d.y", plrno, i);
     secfile_insert_str(file, pcity->name, "player%d.c%d.name", plrno, i);
diff server/unithand.c server/unithand.c
--- server/unithand.c	Mon Oct 16 10:38:32 2000
+++ server/unithand.c	Tue Oct 17 11:31:03 2000
@@ -155,7 +155,7 @@
   struct city *pcity;
   
   if(!(punit=player_find_unit_by_id(pplayer, packet->unit_id))) return;
-  if(!(pcity=find_city_by_id(packet->city_id))) return;
+  if(!(pcity=find_city_by_orig_id(packet->city_id))) return;
 
   if(punit->x!=pcity->x || punit->y!=pcity->y)  {
     notify_player(pplayer, _("Game: Illegal move, unit not in city!"));
@@ -190,7 +190,7 @@
 		       struct packet_generic_integer *packet)
 {
   struct player *pplayer = pconn->player;
-  struct city *pcity=find_city_by_id(packet->value);
+  struct city *pcity=find_city_by_orig_id(packet->value);
   struct unit *punit=find_unit_by_id(packet->value);
   struct packet_generic_values req;
 
@@ -223,7 +223,7 @@
 {
   struct unit *pdiplomat=player_find_unit_by_id(pplayer, packet->diplomat_id);
   struct unit *pvictim=find_unit_by_id(packet->target_id);
-  struct city *pcity=find_city_by_id(packet->target_id);
+  struct city *pcity=find_city_by_orig_id(packet->target_id);
 
   if (!pdiplomat) return;
   if (!unit_flag(pdiplomat->type, F_DIPLOMAT)) 
@@ -306,20 +306,21 @@
   struct unit *punit;
   
   if ((punit=player_find_unit_by_id(pplayer, req->unit_id))) {
-    struct city *pcity;
-    if ((pcity=player_find_city_by_id(pplayer, req->city_id))
-	&& pcity->owner == punit->owner) {
-      unit_list_insert(&pcity->units_supported, punit);
-      city_refresh(pcity);
-      send_city_info(pplayer, pcity);
+    struct city *new_homecity = 0;
+    struct city *old_homecity = 0;
+    if ((new_homecity=player_find_city_by_orig_id(pplayer, req->city_id))
+	&& new_homecity->owner == punit->owner) {
+      unit_list_insert(&new_homecity->units_supported, punit);
+      city_refresh(new_homecity);
+      send_city_info(pplayer, new_homecity);
 
-      if((pcity=player_find_city_by_id(pplayer, punit->homecity))) {
-	unit_list_unlink(&pcity->units_supported, punit);
-	city_refresh(pcity);
-	send_city_info(pplayer, pcity);
+      if((old_homecity=player_find_city_by_id(pplayer, punit->homecity))) {
+	unit_list_unlink(&old_homecity->units_supported, punit);
+	city_refresh(old_homecity);
+	send_city_info(pplayer, old_homecity);
       }
 
-      punit->homecity=req->city_id;
+      punit->homecity=new_homecity->id;
       send_unit_info(pplayer, punit);
     }
   }
@@ -910,7 +911,7 @@
       && punit->homecity) {
     struct packet_unit_request req;
     req.unit_id = punit->id;
-    req.city_id = pcity->id;
+    req.city_id = pcity->orig_id;
     req.name[0] = '\0';
     return handle_unit_establish_trade(pplayer, &req);
   }
@@ -940,7 +941,7 @@
 
     /* if is_diplomat_action_available() then there must be a city or a unit */
     if (pcity) {
-      packet.target_id = pcity->id;
+      packet.target_id = pcity->orig_id;
     } else if (pdefender) {
       packet.target_id = pdefender->id;
     } else {
@@ -1129,7 +1130,7 @@
   if (!punit || !unit_flag(punit->type, F_CARAVAN))
     return;
 
-  pcity_dest = find_city_by_id(req->city_id);
+  pcity_dest = find_city_by_orig_id(req->city_id);
   if (!pcity_dest || !unit_can_help_build_wonder(punit, pcity_dest))
     return;
 
@@ -1176,7 +1177,7 @@
     return 0;
     
   pcity_homecity=player_find_city_by_id(pplayer, punit->homecity);
-  pcity_dest=find_city_by_id(req->city_id);
+  pcity_dest=find_city_by_orig_id(req->city_id);
   if(!pcity_homecity || !pcity_dest)
     return 0;
     
@@ -1489,7 +1490,7 @@
   packet->owner = punit->owner;
   packet->x = punit->x;
   packet->y = punit->y;
-  packet->homecity = punit->homecity;
+  packet->homecity = orig_city_id(punit->homecity);
   packet->veteran = punit->veteran;
   packet->type = punit->type;
   packet->movesleft = punit->moves_left;
@@ -1510,6 +1511,6 @@
   packet->carried = carried;
   packet->select_it = select_it;
   packet->packet_use = packet_use;
-  packet->info_city_id = info_city_id;
+  packet->info_city_id = orig_city_id(info_city_id);
   packet->serial_num = serial_num;
 }