"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."
The DDP is all or nothing: its for everyone on the room reservation and for the full stay of the reservation. I wasn't sure if you were just taking one person off the reservation.
The daily cost for the plan is $38.99 Adult and $10.99 Child(3-9).
Your TS meal would need to cost $32 match the daily cost of the plan. It's pretty easy to spend that for beverage, appetizer, entree and dessert at most TS places at Disney(alcohol not part of plan). If you don't spend that on your daily TS, you still have a CS and snack credit to close the gap to match the $39 per day budget.
Hope you get the trip that you want!