Display Name, I believe, is pretty much legacy from the last version of the website. We really only use Username on this version except on a user's profile page. We've built the functionality to allow users to change their own username but it hasn't been pushed up live yet. There was a bit of a snag last week - we had to make it work with VS Achievements which is tied to username but that was fixed so I would expect it to be pushed up live soon. In the meantime, you can email us with your old username and what you would like it to be changed to.