To answer the second question first, to add dining and tickets changes a room only reservation to a package. They will book a new package, and cancel your room only reservation.
Which brings me to the first question - generally, you will not be charged a fee to change a room only to a package. Remember that changes to room only reservations can be made up to 5 days before arrival. However, it is possible that 1) the deal you booked was only available for a room-only or 2) the deal is no longer available, and since they have to re-book, you may lose your deal.
If you think you may want to turn a room-only into a package later, it may be worth it to book a package initially by adding a one-day ticket. That will require a $200 deposit, but would allow you to make multiple changes.