Ok, this isn’t new, but Windows has the memory of an elephant, and remembers serial dongles that no longer exist. I was up to COM21 and only had two external USB-serial dongles, so it was time for housecleaning.
Found this page that absolutely was spot-on and that I’d never have divined on my own. I think it was the combination of running Device Manager as Administrator, and that’s apparently not straightforward to do (even though I have admin privileges).
After deleting 6 GPS serial instances, 8 FTDI instances, and another 5 Prolific instances, I rebooted the box, waited for it to find the two USB-serial adapters, and now am happily back to COM3 and COM4.
Small victories are good.