We used to be able to have an open booking- just deposit $100 and you could have a future booking. No more.
Now, you book and deposit for a future cruise, which you can change as long as it's more than the 90 days out.
Make sure your docs state ON BOARD Booking. I had one for 2006 that I re-arranged to a 2007 and they said that I did not deposit onboard because there is no cruise consultant on the PG. You get a letter to give to your TA, which I sent to Ngaire. Anyway, she fixed the whole thing, but RSSC forgot that the PG on board is really within 10 days of the cruise.