Please keep in mind that cruise lines offer different "Specials" during the booking time for any given cruise. For instance, one of the other passengers may have booked the cruise before you or after you when the cruise line was offering a different promotion than what you may have gotten for the same cruise. Although for most lines, these promotions are "For New Bookings Only" for the time the special is running. Some lines "may" be able to change the promotion but when you gain some of the new offerings under the different promotion, you may also lose some of the offerings afforded on the promotion you are giving up (ie. you may have received a free specialty dinner onboard with the original promo but with the one you are changing to, you may lose it  but will be gaining something else with the new promo). These promos can be things like upgrades, OBC's, speciality dining or even a free excursion. This happened to us on our Princess sailing this last April. I lost 2 specialty dinners by changing the promo to a newer one but was upgraded from an Obstructed Oceanview to a Balcony for no additional cost when Princess was running a promo different than the one they had when we booked months earlier. All in all, it depends on the promotion running at the time of your booking.