You can call (or your TA can if you booked through one) if it comes out and see if it would save you any $$$ - based on last year's offer you had to pay rack rate (no discount) for the room, everybody in the room had to be on the same plan, at least a base ticket for everyone in room, etc. so sometimes it was not less $$$ to go with
free dining and sometimes it was much, much better to take it, based on your resort choice and how many people are on your booking
A lot depends on what you have booked now (does your booking have any room discounts now?), and what they may offer, it may or may not be cheaper to go with free dining - also you might want to have a second room choice ready in case the available rooms for free dining are all sold out by the time you call - yes, they do go that fast! Disney VISA cardholders were offered the deal first on April 1, then everyone else on April 4, so the cardholders had a 3 day head start. But last year, Disney DID extend the offer expiration so they must have had some cancellations.
Free dining for this year was already offered to UK residents and the "bounceback" offer was already there for people staying at a Disney resort last November - December 2006, so room availability might be tight this time, if they even offer it.
But anyway, to answer your question, I have changed my reservations when a deal came out and they just credited my credit card back, no problem-o!
