FYI, room only reservations can be booked 499 days out by phone, and then once they release the actual rates for that year, online. You don't need to wait.
What they do before the new year's rates are released is the reservation is done and deposit taken at the current year's rates and then adjusted up or down once the actual rates are released.
We booked our fall 2019 trip by phone as soon as we were able to at the 499 day mark as we wanted to ensure we had the room type we wanted. While it would be nice not to have to pay rack rate, we booked a resort and room type we would be ok with paying rack rate for if no discounts become available [which we figure is likely with SWGE].
If a room only discount later becomes available and
MDE will let you, and there is availability, you can change your reservation to it online [by doing a change/modify reservation NOT//NOT a cancel] or by calling Disney.
Note that just because your resort is included in those with the discount, doesnt mean there will be availability for the dates or room type you want -- they are often very very limited in # of rooms that the discount can apply to. And it is not automatically applied, you have to actively have it applied to your reservation.
If the discount is for a package, then you need to make a new reservation and would then cancel the room only. I recommend making the new reservation first so you know you have it and then cancel the one you no longer need -- you don't want to cancel first and then find there is no availability left at all !
SW