Opening the WebCase turned out to be good advice (I think), although I can't be sure to what extent it was responsible for what happened. I received a form-type letter from Adobe by email. It contained instructions which I was unable to comply with fully in the order presented, but I carried out those steps which were possible, de-authorizing with Cmd-Shift-D the old version of Digital Editions (which I had previously restored without solving the problem). I then opted to Upgrade once more next time I started DE. (under Macintosh OS X 10.4 BTW). Everything went smoothly. It didn't ask for my AdobeID nor require that I authorize the newly installed version manually this time. (On my first attempt it had left me with no option but to start a new AdobeID since the online component refused to recognize my email address. The new (and apparently different) ID is what made it incompatible with my ebooks).
I do know by now that my original problem was caused by not knowing the original AdobeID (if indeed I was ever issued with one). I still can't find it, but at least I am no worse off than I was before upgrading. I am not sure if my new AdobeID will work if I ever try to upgrade again, but at least I now know what the problem is and how to fix it if it doesn't.
My advice to others experiencing similar problems: Remain positive. Don't give up. There is good relevant advice in thread "Error: document licensed for a different user account" (IIRC). Start your own Web Case if you reach a dead end.