I made the mistake of running
Now, when I reach the GUI logon screen and attempt to logon with the new name using the password of the old name, it seems to accept the password, the screen blanks as per normal as if the Xserver is about to start, but then returns-- without an error message-- to the GUI logon screen.

There, I selected "console logon" from "menu", logged in from the command line as root and successfully ran
and the procedure ran nominally without error.

Returned to the GUI logon screen and attempted logon as newname with the password that had just been set. Same result as mentioned above with GUI logon screen. Again selected "console logon" from "menu" and successfully logged in as newname with the password that had just been set. Ran
After a long wait, it returned the error message
followed by
It ran the latter message in a loop several times, then screen went blank as if Xserver was about to start, but then returned to the command line environment with several error messages about fonts, followed again by
Trying to login from the GUI logon screen as oldname results immediately in the error message "login failed".

Navigating from command line confirms the directory /home/oldname/allfiles still exists, so thank goodness my data is preserved and safe.

Can anyone here please help this poor sap who screwed himself?!? Is there some trick to logging in from the GUI logon screen as newname that will succeed? Or should I log in as root from the command line and try to reverse the name change I did that started this whole mess?
Please don't hesitate to let me know if more information is needed. Thank you!

Basically just started all over.

Logged into the command line as root, used "groupdel" and "userdel" to just get rid of everybody. Then used "adduser" to restore OldName and everything boots into GUI just fine-- even my /home/OldName directory is intact, as well as all my auto-start items.

Sometimes I think I tinker too much with Linux!