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Verify you have a known working disk password if the system is encrypted!
Login to the computer using your M-1 account.
Launch the Terminal app.
Type "sudo dscl . delete /Users/oldname" where oldname is the user's previous uniqname. This removes their account from the local directory service.
(if needed, CAUTION see notes below) Remove the home folder whose name matches the new uniqname by typing "sudo rm -R /Users/newname/" where newname is the user's new uniqname.
Type "sudo mv /Users/oldname/ /Users/newname/"
Type "sudo chown -R newname /Users/newname/"
Type "sudo rm /Users/newname/.account"
Restart the computer and have the user login.
Additional Notes
Step 4 5 (removing new name) is normally not needed because if the user has been unable to login with the new uniqname, it should not have created a home folder for that uniqname. It is possible that other trouble shooting steps have created this directory, and in that case it should be removed. Exercise caution by making sure that the folder you are removing does not contain the user’s data. All of their data should be located in the home folder with the old uniqname at this point.