Thank you for the help.
I have been following the prompt that comes up in Calibre to update and have been downloading from the Calibre site and installing from my downloads folder.
That must have been a one time accident with Build 8.47
Since I'd been upgrading for three months up to that point, it wasn't a matter of not reading the instructions. Occasional accidents happen and you've got to admit the error message didn't give a very good clue as to what had happened. Since the software kept working (until the Google Play update to Kobo Vox), I had no reason to think things had gone poorly on one install back in April.
Anyway, it's fixed now and I've got my library updated.