First off, thanks again for your suggestion, Eddy.
At the risk of being perceived as unappreciative, as I was about to uninstall Avast Pro using Geek Uninstaller (the free version, IMO a superior utility) I right-clicked the Avast Pro entry on the list so as to reveal the uninstall option. To my surprise I noticed a "Modify" option listed. Clicking it revealed a further alternative to revert back to my previous Avast Free. Sorta-kinda like a restore point. This was what I chose. After a bunch of Avast system file changes were completed came the recommendation to reboot, which I did. One of the first things done after logging on as administrator was to verify my Free install status according to Avast. It disclosed that all previous account data was still intact, which included the results of the last full system virus scan, with the same four files stored in the virus chest.
I've got my fingers crossed that the reverting to the prior program will remain and persist, but I can say with certainty that this unforeseen option with Geek uninstaller surely saved an enormous amount of time in recreating a mirror image of my settings prior to upgrading (and then changing my mind) on Avast Pro.