They could "simply" be made unique by merging their character ID# with the continent ID# they are currently on. No possibility for doubles this way. A new algorithm would need to be made to generate new characters' ID#, but I'm under the impression this would be trivial as well.
Indeed, I would imagine Tom, Tim, or Peter could modify the game DBs and generate unique character IDs in a matter of a few minutes. They are strong in SQL-fu.
Granted though, changing all references to character name for character ID in the code probably isn't that trivial a task at all, however.
Which is really the crux of the problem. Designing a robust system that can track unique IDs over the many separate world DBs, and then retrofitting all the game code to deal with it. I doubt that this is something that could be done on a stable/testing basis. (IOW it would have to go live across the entire game, all at the same time.) The potential for havoc in the event that undetected bugs arise is also very high. I don't
know that this would be a coding nightmare, but I have a very strong suspicion.