OK, testing is finished.
As expected, adding PID.txt (Win 8.1 product key) to the flash drive gives me Windows 10 Pro.
I fully expected the generic key to give me Windows 10 Pro too, but it did not. When the install finished, I only have Home. So that's a bummer. I did double check to be sure I had exactly the key I should have.
For now, I have other things that must get done, so any more testing will have to wait.
Thanks for the help with testing.