Once the guest has been given the room number, yes, it's highly unlikely to change. But before that, even if online check-in has been done, the pre-assignment is tentative. If another guest shows up and wants a different room, and the room you've been pre-assigned to looks like a good fit for that guest, and you haven't turned up yet and the room number hasn't been sent to you, there's no reason they can't switch you out of that room and put you in a different one. Happens all the time.
The story we've always gotten from the resorts is that nothing gets put in the room before the guest's arrival. I've heard that it's sometimes done for club level guests, but that's a much smaller, self-contained set of rooms.