Pokemon Pokeball Replacer   Basically, in Pokemon Summary window, an option for players to "replace" current pokemon pokeball.   "Are you sure you want to replace pokeball with GREAT BALL? Replacing pokeballs will permanently delete your current pokeball" can be the dialog for such a system. OR  You can make it so the current pokeball is not lost during the replacement process.  I KNOW what you are thinking next, well wouldn't this make master balls extremely abusiv