As expected "Full Self Driving" means the driver has to supervise at all times, so no sitting in the back reading the newspaper.
So very limited benefits in my view.
Whilst not 100% certain, I don't ever expect existing Tesla owners will get the kind of "Full Self Driving" that does allow the customer to sit in the back, because as far as I can tell, they don't have the necessary redundancy in steering and braking systems.