I'm not sure what you mean. My username is the one I use to log in. And the @handle part of the character@handle name.
Unless the signup has changed, there was a warning about using the same player name as your account name. So "Rocketeer_7" is my player name and it's much different than my account name.
Someone mentioned changing names. You can change a character's name by buying a token from the C-store. I think you can change your account name, but it would be a pain. You'd probably have to explain to Cryptic/PWE that you suspected that someone was trying to hack it.