Remember when you book a cruise on board you know at that time what the discount will be 5% or more.
Although they have Phantom bookings it is easier to keep track of an onboard deposit when it is tied to an actual booking number. Those open bookings honestly were a nightmare to keep straight and I must admit although I did not initially like these new rules it is easier when an onboard is tied to an actual booking.
At this time Radisson will on a case by case basis retroactively apply the 5% to a booking already made as long as the fare has not increased and the same category is still available to be booked. Obviously as hrprof mentioned you can just cancel and rebook anyway.