Your ticker says your trip is in one week so if the question is about this trip there are some deadlines to be aware of. You would be booking a ticketless package, not adding a dining plan to a room only reservation so the entire cost of the package will be due when you book it. Dining plans must be added to a package no later than 3 days before arrival.
However, the deadline for cancelling a room only reservation and getting your deposit back is either 5 or 6 days before check in date. (I can't remember the exact wording.) They will not transfer your room only deposit to the new package res.