I've booked with F2F a few times, but always ended up canceling. Each time, I would have my preferred flights booked and paid for, then Carnival would change the flights, but blame the airline(Price on the flights went up before Carnival secured them) from direct flights to flights requiring a layover, adding hours to my flight. I canceled and rebooked the exact same flights direct with the airline for the original price I paid Carnival.
Note, if you book a flexible option with Carnival, you can make changes up to 60 days before you cruise.