The way I understand it, you can't change your account name, but you can create a new one. The info associated with the old account gets permanently deleted after some amount of time. I believe messages after 3 months and transaction information after 4. At a certain point, like 4-6 months after your last login, it gets "archived", whatever that means. The only thing I know about it is that you can't log into it anymore. It appears to be deleted. At that point it should have no info associated with it anyway. Make sure you remove ALL BTC from the account and don't log into it.