How do I get the screens to be interchangeable again from horizontal to vertical??
As you haven't already stated it, can you confirm whether or not you've checked the rotation lock setting? (Double-tap the Home button and swipe right to find it)
I have done that but and it locks either way. It won't interchange anymore when unlocked. It's either locked horizontal or locked vertical. Even with the rotation lock not locked it will stay horizontal.
Might be a faulty orientation sensor.
Firstly, I would do a reboot, then a Restore from backup, then if all else fails, a restore as new.
However, to give you a clue in the meantime, you can donload a free app that will check your orientation sensor: "iDemonstrate"
Retrieving data ...