On any forum software that I've seen, members can't change their own usernames. Some, however, have a function, that an admin may or may not have enabled for the members, for a user to change the display name, which is distinguished from the username--the username is what you log-in with and what's displayed everywhere, while being able to change the display name will change what members see in posts, but not what you log-in with. I'm sure all have a function where an administrator can outright change the username.
Having said that, the admins here have most probably deemed this to not be a feature fit for this message board for whatever reason. However, the banned member WarioBarker became brtsimpson142--a full username change--at one point by what I gather to be a request to an admin, though it might have been a topic like this one. I'm not sure. I do know, though, that, based on this precedent, you should be able to have your request honored as long as you're reasonable and ask nicely.