Woohoo! Success! Removing those files from upgrade_changes.txt did the trick. The upgrade script continued on its way.
I then encountered
this problem with image libraries. Using Ruben's advice there, I updated each image setting in the Control Panel / Styles section.
Running happy as a clam on PHP 7.3. I still have to update the forum view settings for non-admin users. Then it's off to push from my test machine to the live site.
Thanks all again for the help!
jjjjj, I hope you have similar success and I apologize for taking this thread over.