You can book a "Next Cruise" on Celebrity Cruises while onboard a Royal ship (I have done this), but once booked and the money has been paid, it can only be changed if you purchased a refundable deposit and by canceling it before the final payment date and then rebooking. Although they do have the same parent company, they are still separate corporations. Once the money is on the books of one company, it would be an an accounting nightmare to switch it to the other.