Princess will even let you book an "open booking". ie: make a booking, get the incentives, but you don't have to specify what ship or exactly when you will cruise, as long as it's within the next 4 years! What a deal. If you're like the lovely Mrs. Jones and me, you know you'll sail Princess again within that amount of time. (probably several times in the next 4 years!) They only require a $100 per person deposit, and you can transfer the booking to your favorite agent. It's a win win deal!!!!!
See you aboard,
Sadly Carnival will not. Tried on Glory and was told Carnival didn't do this. Oh well. But have successfully booked RCI cruises while on Celebrity and Celebrity cruises while on RCI. And then as Kuki said transferred them to my TA.
Not only did Princess allow us to book onboard, but they gave us a certificate so one other stateroom could be booked at our price and with the OBC and $100 deposit. Really nice to use when twisting a friend's arm to go with us to the British Isles.
About the Sea Miles program. It's a reallll sneaky program and I'm quite disappointed in how it works (or doesn't work to be more specific). But that needs to be another thread, I guess.