I encourage you to test this out yourself using a real device. I believe that what should happen is that the location permission should be the same that was before the phone were upgraded to iOS 13, but if you test this yourself you will have more confidence about it.
I'm in the same boat.
Furthermoe, If the permission is indeed stay as "Always" after upgrding to iOS13 - Is it Provional Always or Constant Always?
Did you manage to find an answer for that?