you would need to change either the tid, sid, or pid. you probably want to change the pid, as the other 2 associate the pokemon with you. this is from my ram watch:
00012 00000000 S S 0 ----------------------------
00013 022349D4 d h 0 Slot1 PID(W)
00014 02234AB0 d h 0 Slot2 PID(W)
00015 02234B8C d h 0 Slot3 PID(W)
00016 02234C68 d h 0 Slot4 PID(W)
00017 02234D44 d h 0 Slot5 PID(W)
00018 02234E20 d h 0 Slot6 PID(W)
00019 00000000 S S 0 ----------------------------
0001A 022349B4 d h 0 Slot1 PID(B)
0001B 02234A90 d h 0 Slot2 PID(B)
0001C 02234B6C d h 0 Slot3 PID(B)
0001D 02234C48 d h 0 Slot4 PID(B)
0001E 02234D24 d h 0 Slot5 PID(B)
0001F 02234E00 d h 0 Slot6 PID(B)
00020 00000000 S S 0 ----------------------------
basically, put the poke whose pid you want to change in your party and reset the pid using these. for example, say you're using white english and you want to change the pid of the second poke in your party. you would make up a pid using pokegen or something and use that as the new pid, so you would end up with something like this:
02234AB0 1234CDEF
that changes the pid to 1234CDEF in hex. once you do that and start the game, go ahead and save the game and you'll be set.