There is, indeed, a way to change your username without creating a new account! Community Central explains it better than I could, though, so if you want to see how CC explains it, click this link. If that's not clear enough, I can try to help further.
It's saying I'm not allowed to because the name I'm choosing is trying to impersonate someone else. I'm not sure what that means, there's no one by the name LeifEricson .
