I figured it out.. just Another weird Adobe bug. Even though I'd replaced the Tag with the new one (and thus the new one was the only one in the Property/Subject field), the old one was also listed in Calibre. But now (maybe after an Acrobat update>I'd changed the tags a few months ago), if I delete the new tag, save the pdf, and put the new tag back, Calibre now just shows the new, desired tag.

Thanks for checking in, I really appreciate it.

Any chance the 'Polish Book' thing could be available for PDFs.. would be a great way to just adjust title, author, tags without reconverting.

Thanks for the great app, sir... truly a work of art.