In the past, we have booked a GTY inside stateroom CAT11 and after final payment, before sailing some time they gave us a free upgrade to a CAT9 oceanview room. That was one heck of a pixie dust upgrade for us!
So, it does happen, no matter what category you are in...if the ship is not filling up to Disney's standards, they will upgrade those who have already booked to make room in the lower categories for potential bookings.
I have even heard of people getting free upgrades when they have an assigned stateroom.
Good luck to you!