We have always had success changing an ADR, either changing the time or changing to a different restaurant, on the day of as long as we were just changing and not cancelling.
Example. We drove to and parked at Kidani for dinner at Sanaa. Walked over to Jambo to kill time before dinner. Decided we wanted to eat at Jiko instead as I saw an open time slot in
MDE. Jiko CM booked the new time for us and called Sanaa to make sure we wasn't charged the cancellation fee.
Obviously they have the right to say no at any time but it hasn't happened yet. But we always ask for things within reason too.