If you book online at Disney's website for full price and then later hear about a code ..... can you call them and get them to apply it to your reservation?
Yes, but watch out for the cancellation procedures on the site. I seem to remember a thread recently where someone booked through the Disney site and the cancellation procedures were more stringent.
Other than that, there should be no reason why you wouldn't be able to modify your reservation to a better rate (should it become available).