The easiest way to change a user name in a Windows NT 4.0 domain is to use tip 3072 » How can I rename a UserName from a batch or command prompt?
To change the user name using User Manager for Domains:
1. Highlight the account you want to rename and press Copy on the User menu.
2. Type the new name in the User Name box and complete the dialog.
3. Check User must change password at next login and press OK.
4. Double-click the old account and set Account Disabled and press OK.