AG3 will probably let you change your username to something which another player hasn't already chosen (for those that ask, there *is* a player named '1337' on the system already).
The old AG2 system ties more information to your username than a unique identifier for your account, so changing your name on AG2 became a nearly impossible feat. On AG3, we're making sure to do everything with the unique identifier (which is common practice, and just plain good design, in my opinion), so a name change will be much easier.
The only thing is whether or how we notify other users that you've changed your name. Maybe we'll overlay your armatar with a small icon for a few weeks that indicates you've changed your name or something.