How difficult will it be to reconfigure a Starliner between flights? Will we be able to do one flight carrying High Class seats then change out the seating to be mixed class for the next flight?

It will not be especially difficult; you'll modify the internals of the ship using a holoviewer style interface much the same way you modify exterior hardpoints. Note that you won't, however, travel with an unlimited supply of alternate seats, cabins and the like: you'll need to plan in advance to make sure you have the needed stock at your destination.