I never had to do it before, so I don't know how it used to be done.  Looking at it now, I don't see how to do that with the new site.  I would assume that you would have to either contact the Admin here to change it and keep all your old posts/reviews/messages/Profile info or if you don't have anything to lose, just open up a new account with the Username you really want if it is available.

Hopefully, someone who does know how will reply.