Technically, the Room-only and the Packages are in two different systems. Your reservation # will remain the same, but it's like having a whole new reservation. In effect, you are canceling and re-booking, whether you go from a package to a room-only, or room-only to a package.
If a PIN or other discount comes out and you want to change your reservation,it will depend on availability. Each deal has a certain (unknown) number of deals available at each resort and in each room category at each resort. My recommendation is to book the room category at the resort you most want. If you get a PIN or other discounts become available, call and see if your preferred room type/resort are available. If so, then switch (it's easy, I've done it several times). If not, then you will have your preferred resort. If it's worth it to you, then you can always switch to another room type or resort that has deals available. Clear as mud? lol
For example--last year I really wanted to stay Poly CL for my birthday trip. I booked about 10 months in advance. When a discount became avaiable, I called right away as soon as I heard. Fortunately, I was able to save 40%! I got lucky. However, if Ply CL hadn't been available, I still had my dream room. (Although I would probably have switched for a 40%, but....)