Sorry, I guess it's $100 if you cancel, $50 if you change:
Changes to Reservation - For a change made 44 days or less prior to guest arrival that results in a change in the package price, a change fee of $50 per package, plus any change fees assessed by hotels or other suppliers, will be charged. Disney reserves the right to restrict changes to any reservation. Such change fees must be paid in addition to any increase in the package price resulting from the change. If air travel has been ticketed, guest will be responsible for any increase in airfare as well as the change fee assessed by the airline.
Maybe depending on who you talk to, they won't charge you the $50. However, since adding dining is changing the package price, technically though they could charge you.