Oh my bad, now I see. Right now the way to do that is to just create a new account with the new domain. Roadmap: I'd like to have the ability to add (or remove) domains under one Organization. Signups are disabled right now while I'm fixing a spam attack, but I can help you setup a new one
Are you wanting to change your username? I can do that for you (still working on the settings page to do that). What do you want it changed to?