It likely would not be that simple considering OLED is a completely different screen technology, so in that context it would make more sense to do a redesign. It doesn't have to be a huge redesign, but it would be a redesign nonetheless. Furthermore, the non-Pro non-Air iPad already has a landscape camera, with magnetic Apple Pencil (USB-C) support too, so it would be very strange for that machine to have it while the flagship Pro machine does not. Plus, the current design is already in its 6th year so it's due for an update.I don’t see a landscape orientation camera coming.
Apple will want to keep costs down by continuing with the same chassis and simply replace display and chips.
Personally, I think there is about a 95% chance that when the iPad Pro goes OLED, it will have a new form factor with landscape camera. And with that, we'd get new keyboards as well.