Upgrade to XFG 2.2 and users can change their usernames themselves. As the admin, you can set the options for this to limit how often and/or how many times members can do this, and you can require approval of all name changes (i.e., changes are sent to the approval queue). Then all you have to do is check the permissions box to let moderators approve name changes. (The history of previous names is saved by the way.)