As of now you cannot change your username without getting a new account. When AG3 comes out though you will have a chance to change your username then.
Yes you will be able to change it in AG3.What will happen is,you will choose a username.Then you will wait a month with your old username.Then after a month it will change.
Check out the stickies in the AG3 sub-forum for things AG3 related. There's something about username changing and the release date up there among other things.
