Three months is already generous, since many users here take this duration as an invitation to flip-flop and be fickle with usernames. Almost any other website with a community wouldn't even give users an option to change their names, so we shouldn't take these opportunities at a name change for granted. :P
To avoid these technical issues in the future, perhaps we should expand with more guidelines - not rules like with the @, +, and / symbols, just suggestions - on how certain characters in a username will affect the profile's URL? That might help as well.