No need to reimage.
To reverse the situation,
sudo defaults delete /Library/Preferences/com.apple.loginwindow Hide500Users

This will make the account visible in the Accounts preference pane unless you had added the HiddenUsersList dealie someone else suggested, in which case:
sudo defaults delete /Library/Preferences/com.apple.loginwindow HiddenUsersList

should do the trick.

Log in as a different admin (create one if necessary?) and control+click the previously hidden user icon in Accounts pref pane and change the user account ID, and/or home folder location.

If you change the uid, you will need to chown the home folder to the new uid by:
sudo chown <adminusername> /path/to/home/folder

If you changed the home folder location, you'll either move the old home folder to the new location, or allow the system to create a new home folder when you log in as the previously hidden admin user (if you created a second admin account that you no longer need, delete the second admin account through Accounts pref pane) and dispose of the old home folder.

I hope that's helpful. It can all be scripted also (though I doubt there would be much call for that).