As long as you change at least 45 days prior to check in there are no penalties. However, discounts under certain deals may be limited. Only so many rooms are alotted per resort per discount. For example, last January I booked Sept 24th-Oct 2nd at POFQ under the GP code that came out for most dates in 2011. When we were down there the first week of May, I decided to switch to the bounce back deal that was on a flyer in our room. Those dates were included as part of the bounceback time frame. However, when I called, neither POFQ nor CSR (my two preferred resorts) had availability for those dates. Nor did they have any availability for the week prior or the week after. I had to either change resorts or move the trip back two weeks to Sept 10th-18th, which I did. Even with those dates, POFQ still didn't have availability. I also always go to WDW the first or second week of Dec, but due to black out dates and lack of availability had to move that trip back to Nov 27th-Dec 4th. I still need Nov 26th, but it wasn't included in the bounceback dates, so will have to either pay rack rate or hope for an AP rate for that one night.
In the past I've also had to change resorts in order to use a certain promotion. I know I had to switch from POP to AS Music once and from POFQ to PORS another time. Last Dec I had an eight night stay at CSR booked, but decided I wanted to do a split stay at CSR and WL. WL was not available under the promotion I was using, so I had to do CSR/YC.
Book your airline while the rates are good!
