For those of us who are reading this it's way too late to worry about changing our username, but the reason I'm suggesting it is because I'm in a game with a new member and apparently their username isn't quite what they wanted it to be. They asked me if there was a way to change it.
I advised them to send a feedback and see what happens but not to get their hopes up. I have not heard back if they sent one or if they got a response. So far the username has not changed.
In the early days of a persons membership it should not be much of a problem if they messed up and want to revise their username. Once they get some games completed then it should no longer be allowed at all.
Maybe they could have the option to change it until they have a regular rating, after 20 rated games. Or if that's not good, then just give them a set number of days to make the change. There could also be a limit of one change. A note could be added to their profile page to show the name change in case someone should get confused.