Unless you define a user name or UID number that conflicts with an existing one, you should never need to modify a user account's login name or UID number. Use the following steps if two user accounts have duplicate user names or UID numbers:
If two user accounts have duplicate UID numbers, use the Users Tool to remove one account and re-add it with a different UID number. You cannot use the Users Tool to modify a UID number of an existing user account.
If two user accounts have duplicate user names, use the Users Tool to modify one of the accounts and change the user name.
If you do use the Users Tool to change a user name, the home directory's ownership is changed, if a home directory exists for the user.
One part of a user account that you can change is a user's group memberships. Select Properties from Users Tool's Action menu to add or delete a user's secondary groups. Alternatively, you can use the Groups Tool to directly modify a group's member list.
You can also modify the following parts of a user account:
Description (comment)
Login shell
Passwords and password options
Home directory and home directory access
Rights and roles