During past promos, I've been told specifically that it is possible to change back and forth (including by one of the Marriott Concierge reps), but you need to specify one or the other by making the switch in your account preferences prior to the stay (and it is best to wait until the points or miles post before trying to switch back, if that is what you want to do). I was also told you could alternatively ask the hotel to hard-wire (there's a specific term they have for this which escapes me at the moment) it for miles or points earning at check-in for that specific stay - but I never tried to do that. I did however, change back and forth maybe 3 or 4 times during that promo period and ended up with some stays earning points and others earning miles according to the way I wanted).