I'd say it must be easy to do because I had to cancel a guest from my reservation way after the PIF date, and I was told I could switch the reservation to another person and maybe just pay a small fee to do that (since it was after PIF).
Changing the name of a guest before the paid-in-full date should be easy. If you've booked through a travel agent, the agent will need to contact Disney to make the change. If you booked on your own, you can call and get the change made.
If the fare for the cruise has gone up since you originally booked, it is possible that the new guest will be charged the current fare and you would need to pay the difference, but I'm not sure about that.
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