Can't change short name of Administrator


I just installed OS X for the first time.

As all of you know, the installer asks you for the information to set up an account. I gave my name and my first name as the short name.

Only later did I realize that I really wanted to have that account as an administrator account and wanted to use that short name for my regular user account.

When I went to the user manager to change the Administrator setup, the short name was grayed out. Why is that, and how do I bypass that?
log in as root (of course activate it first through the utilities folder), and then to try to modify the administrator name...