I believe the technical issue is that Marriott doesn't allow any changes to awards. So if you want to change the award, the mechanics of it is that you are actually cancelling and re-booking a new award...hence the price difference. I've also run into this situation where i can't change an award booking because the hotel is sold out. The normal way a phone rep changes an award booking is to book the new one then cancel the old one...but if no inventory they can't book the new one and cancelling the old one runs the risk of your room not going back into award inventory (or someone else scooping it up when it does become available).

Bottom line is the IT system does need to be upgraded to enable true "changes" in award bookings.