After installing Mojave, my previous username/password combination does not work I just installed Mojave on my MacBookAir, and now my username/password (under Yosemite) are not accepted. I called Apple support and they told me to restart and immediately push down command-r giving me a MacOSUtilities window. I, then, chose the DiskUtility option that was offered, but the advisor didn't know what further to do, and scheduled me for a further call tomorrow.

Any immediate advice (somewhat disappointing--one would seem to expect smoother installation behavior from Apple)? Perhaps I'll have to take it to an Apple store for assistance.

Do I need the original "admin password"? (The laptop was purchased several years ago. I don't know if offhand.)
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…