Disney has 2 types of reservations, room only and packages. The numbering systems are different (room only reservations have 12 numbers, packages have 1 number, 2 letters, 5 numbers) the deposits are different, the payment deadlines are different.
Room only reservation numbers look like this 123456789112
Package reservation numbers look like this 1AB12345
.
If you want to apply a discount to an existing reservation and change nothing else, your reservation number will stay the same, and any moneys you have paid will still show as paid and you'll get confirmation showing your new, lower amt. due.
The glitch comes w/ changing from a room only to a package or vice versa, in that case you'll get a new reservation number, you'll typically have to put a new deposit down and your previously paid moneys will be refunded to you (although I've read of cases were people were able to have the moneys transferred from one type to the other.)
Just having a reservation does not guarentee that a new discount can be applied to your existing reservation, if there is availability w/ the discount (eg: your room type is part of the discount offer) then you can have the discount applied.
As a pp mentioned, checking online through the discount link on WDW's website is a good way to check to see if your room category/resort has rooms available under any new discount offers. However, it doesn't hurt to call and ask a CM to check as well (I did this mon. and even though what I wanted hadn't shown up on the computer, the CM found availability.)
Also, it helps to keep checking - what I wanted was not available initially back in 2010, but several weeks later it became available.