After installing Mojave, my previous username/password combination does not work
Easiest: boot from another installation that has good admin access and delete this file:
/private/var/db/.AppleSetupDone
(It supposedly can be done via Single User Boot (Command-S) and deleting with the "rm" command, but that did not work for me.)
You will then get options as if installing OSX the first time, which will permit you to add a good Admin User – which should remain unaltered (in case of future problem) – and that user can change any other user back to an admin…