Why thankee, kind Sir
I created a mirror of the database and the forum itself at a different URL then hit a problem running the PHP patches through. I think the patch command on this Linux system was incompatible with the diff output generated by the phpBB folks, or the files had MSDOS linebreaks in them and that confoozed it. In any event the issue was solved when I used a Windoze version of 'patch' and did the work on my machine, resolved all the rejected hunks by hand, made the schema changes, uploaded the patched files, tested the major functionality and then swapped the live site for the upgraded mirror.
It'll be faster next time, promise
It was the patch failures that had me scratching my head, and I *really* didn't want to do a clean install of phpBB and re-apply all my mods and little bits of restyling. THAT would have been horrific.