[Konstantinos]

pawn Code:

new account_name[MAX_PLAYER_NAME];

GetPlayerName(playerid, account_name, MAX_PLAYER_NAME);
NickToLower(account_name);

printf("users/%s.ini", account_name);

You can convert all the current files from registered players using FileManager. dir_list function to loop through the files of a specific directory, checking if it is a file (FM_FILE) and if it is, rename the file.