Hi Jennifer,
Welcome back to the Disney Parks Moms Panel!
The short answer to your questions is "yes... and no". From what I understand, all
Special Offers, including the "Stay Play and Dine" (i.e. Free Dining) promotion, are often limited to a certain number of reservations at each Walt Disney World Resort Hotel. If you already have a
Magic Your Way Vacation Package booked (regardless of whether it already includes a Disney Dining Plan), you will have to cancel it and re-book it with a Resort that offers the Free Dining promotion. You may get lucky and be able to get a package that is identical to the one you had initially booked, but there is a chance that you may not. In that case, you'll have to choose a different room type/Resort if you want to take advantage of the special offer, or you'll have to stick with your original package (and cost).
Because there's a chance that the Free Dining promotion may be available during your travel dates, I would recommend booking your vacation through an Authorized Disney Vacation Planner. Whenever I've worked with one, they were usually quite good at getting my vacation re-booked seamlessly so that I could take advantage of any special offers that may have been released during my vacation dates.
As you might already know, there are no guarantees that Walt Disney World Resort will even offer the "Free Dining" package in 2016, although, historically this special offer has been available for visits during the month of September. Please keep checking the Special Offers page and keep checking the Disney Parks Blog to make sure that you're one of the first people to jump on this offer if it does get released (or that your Authorized Disney Vacation Planner does so for you).
Have a Magical Day,
Lynne