Sorry Don, but I do need to clarify. There is no hard and fast rule, that gratuities cannot be modified. I have a contact in guest relations at =X= who verified this. Additionally, I just spoke with Brianna and Kirby (sp?) in the Certified Vacation Planning Department (1-800-556-8209) who also verified that this is not a policy of =X=. All have stated that the guest comes first with their philosophy of customer service.
They "strongly suggest" for guests to pre-pay. They also mentioned that the "My Dining" on RCI does require pre-payment of gratuities, but in the end, if a guest wants to modify their gratuity, they can indeed. Again, they stated, this is not a policy which is set in stone, but "strongly suggested". A perfect example would be a pax not dining in the dining room for one-week out of a two-week cruise. She stated this does happen frequently, and they see pax on the last day before the end of the cruise at the purser's desk modifying their gratuities.