Note
whoops. figured I would finally not be way behind and let my MBP update since I oddly had enough hard drive space, only I didn't realize that Big Sur would break parallels. Welp, out $50 to buy new Parallels...only it asks for the original activation key, which I can't find, and can no longer query via Terminal since the old one is now gone.
Two dozen password resets later (solved by resetting pw in a third browser), I got it off the old account, phew.