The terms of a reservation - dates, room rate, any fees controlled by the property, and the cancellation policy - should be set in stone once a reservation has been made and accepted by the hotel. Changes should require consent from both parties. That is the bigger issue that remains unresolved even if this property accepts the request by the OP.

Maybe ​​​​​​Marriott Bonvoy Lurker could chime in with an explanation why their system allows properties to change the terms.