You'll keep the same reservation # regardless of the changes in the reservation - dates, add-ons, etc... Unless you are cancelling completely and requesting a refund of your initial deposit. (Trust me on this one. Unfortunately, I've had to change too many reservations too many times.

)
But having that reservation does not guarantee you a room at that resort with a
free dining (or other) promo. It just guarantees you have a room at the rate you booked.
As PPs have noted, there are a limited number of rooms at each resort that CMs can apply the promo to.
It's kind of like a sale at Sears. They have ## clothes washers advertised for $299 each. When those are gone, that's it. You might get there early and snag one. (Call soon enough for a CM to "cancel" your current ressie and re-book one with the promo.) Or there may be no more sale washers left at your local store (promos left at your resort). Another store may still have some left, but you have to go there to get it. (Promo may still be available at another resort, but not at the one you booked.)