Unfortunately, the Escape package prices are set in stone - they don't consider the elements swappable.
The only thing they will let you swap is, as PrincessBrideKatie said, a honeymoon photography session for the wedding photography that's included in the Escape package. However, the honeymoon session is done in regular street clothes during normal park hours (and you only get 2 prints if you get the session as a swap!), so they don't consider it the equivalent of the MK photo shoot.
Hope this helps! Maybe if enough people complain they will start to be more flexible...