You need to create another temporary admin user, log in as that user, and then open NetInfo Manager. Using NetInfo Manager, change the shortname, AND the home directory property of the user in question. (You'll need to rename the home diretory as well, in Terminal). Once done, log out, and test logging in as the user with the changed name. If everything seems to work, and your home directory is intact, etc., you may delete the other temporary admin user.
Note: usernames ("shortname") and/or numeric uids are NOT designed to be changed after created. This is not "Apple's fault", and there are numerous reasons for this.