The availability of rooms for
free dining has NOTHING NOTHING NOTHING to do with whether or not you have an existing reservation somewhere. Having a reservation does NOTHING to help you get a "free dining" room at that resort or any other resort, for that matter.
For each room category at each resort, Disney has a super secret target occupancy rate ... if the current occupancy rate (for any given date) is below that target, they will include that room category/resort in the free dining promotion, and if that room category/resort is already selling well enough then they won't include it in the promotion.
So, for example, let's say you have a room reserved at POR. And let's say that for your dates of travel that POR happens to be totally sold out. When free dining is announced, NO FREE DINING ROOMS WILL BE AVAILABLE AT POR ... management figures that if it's already selling so well, why lower the price? In that scenario, you wouldn't be able to simply "convert" your reservation into a free dining reservation since free dining isn't being offered at your resort.
This is an extreme example, but I hope it helps illustrate to you that you need not stress over where to make a reservation in hopes that you can later convert it into a free dining reservation ... because having that room with a regular reservation will do NOTHING to help you get that room with the free dining promotion.
If I haven't explained this sufficiently, please post back and I (or someone else) will try to help explain it better.