The best way to get a castaway club discount is to re book while still on board.
I just did this and to my surprise I was given an unbeatable rate. I attempted to transfer my ressie to costco, and they could only match it, not beat it. I also got the $200 onboard credit.
I have never had a situation before where costco did not have the best cruise price.
Way to go disney !!!!
That is their policy though. To maxamize your castaway benifit you have to re book on board. You can transfer the date as much as you need, and still keep the credit.