Part of Sept is the Value season rate category. After Sept 26th you get into Food & Wine season and Columbus Day holiday, and the rates go up. Disney recently moved Food & Wine to start September 19th (from the usual end of Sept start). I think some people going for F&W will want to move into the September Value dates. That's what we did.
Based on moving Food & Wine earlier into September, coinciding with Value rates, my common sense guess would be that rooms will fill up and FD won't be offered. It's only offered to fill hotels when reservations are lagging.
As PP wrote, when you change a room-only reservation to a package, the reservation is actually cancelled and re-booked. Based on info above, there may be no rooms available if you try later to re-book your room-only to package. I'm not sure how that works if you reserve a package with dining plan and tickets, then later cancel dining plan. Is it considered a package modification only, or do you need to re-book? That's a question to ask a Disney agent. Book by phone so you can ask a Disney CM about this. If you didn't need to rebook in the second case, that would be my plan. You are only obligated to purchase minimum 1-day tickets for everyone in the room with paid dining plan. You can add on more days after you check-in. This will save some expense prior to check-in.