Why do you want to change your short name?
You can do this with Utilities->NetInfo Manager. Click the lock and authorize, then click on "users" in the list, select your account and change your name, your home folder etc. Remeber to rename the home folder in the Finder, too, or you'll get a lot of problems. Don't touch the other settings. Log out and in again. Remeber that the short name may not contain spaces or special characters and may not be longer than 8 letters.