[Drebin]

pawn Код:

format(string, sizeof(string), "/userfiles/%s.user", PlayerName[playerid]); //Deletes file with string in "PlayerName"
fremove(string);
SetPlayerName(playerid, tmp);
GetPlayerName(playerid, PlayerName[playerid], MAX_PLAYER_NAME); //Saves user's name into "PlayerName"

If you get the player's name after the old file is deleted, then what file name are you deleting?
Or did you save the name BEFORE the player types /changenick?