Yes if you book a promo during the allocated time period does guarantee you the use of that promo for changes in the future if the changes are available. I booked the bouceback free dining in August 08 for August 09, I just changed my resort from Pop century to OKW with the 30% off and free dining last night.
I was told that since there were 2 adults on my reservation this past august that I could add a second room at the same price. And then I still have until 45 days out to cancel or change again.
Sometimes you have to call a couple of times in order to get someone who knows what to do. As soon as someone answered my call I told them that they were going to have to send me to guest services because it is an offer not available anymore and that helped. One person had no idea so I just said thank you, hung up and called back.
It never hurts to call and ask.