Each KTTW card can be linked to a different credit card. You can do this at check in or later at guest services.
Donna (who does the re-booking on the Magic) is not my favorite cast member. She's given me wrong information each time I've talked with her about re-booking. THe first time, I just walked away and booked on my own from home...got a better deal than her price.
This time, I got internet prices before the cruise. She insisted that they were wrong or no longer available (I doubted that, but...). SO, I booked with her while on board, then on Monday when I arrived home, I re-checked the internet. Found the prices I had seen before on 4 different sites--these were legit Disney prices, not "reseller" prices. SO, I called
DCL and told them what Donna had said and what I'd found on the internet. THey allowed me to change my reservation (keeping the same ressie number as well as the 10% discount and $200 onboard credit.)
I've heard tales of her matching rates that people have printed out, then applying the discount and credit. My suggestion is that you do your homework and print out any offers you find. When booking 6-12 months ahead, it is unlikely that a rate has changed over a few days, but it's always possible.