I was able to make a reservation on the medallion app for our upcoming cruise and it "zeroed out" the cost. My sister who is in the suite with me tried to do it and it said it was going to charge for it, so I think the person who booked the suite/guest 1 might have to do the reservation for the credit to be applied.
In the pre-covid days when we got to our cabin there would be a card with our assigned time and restaurant. If we wanted to change it, we would just call the dine line or stop at Sabatini's or Crown Grill and get it changed. That should still work if the medallion app doesn't