Everyone here is thinking too hard. If you can't change the permissions under the names, change the names over the permissions!
So if Bob is currently the administrator but wants to be a regular user and Sue is a regular user who should be the administrator proceed as follows:
Bob log in as administrator.
Go to Preferences>Users and Passwords.
Select Bob (Administrator) and click edit.
Change "Bob" to "Sue" and set a password to share with Sue.
Select Sue (Selected Access) and click edit.
Change "Sue" to "Bob" and use whatever password you'll remember.
Log out.
Have Sue log in - now as the Administrator - with the password you created for sharing.
Sue can change the admin password once logged in.
Bob can use his regular password to log in as a regular user.
