Call and try.
Last year when I had a 40% off pin, I had booked a week in December for vacation at Disney. However, we found out after the code expired that my dad could not get a week off in December so I called to see if I could make any changes and the CM was able to work with me to still get me the discount for a week in October at the hotel of my choice.
If the CM you get can't help, ask to speak with guest services, they can often help in situations like this. As long as the resort you are switching to, didn't sell out all of it's rooms during the promotion, it may be possible for you to switch.