From Mousesavers.com:
"For phone bookings, no change or cancellation fee will be charged if you make your modification more than 5 days before arrival. You can also book online through the Annual Passholder web site, but for online bookings the change/cancellation deadline is 46 days -- a real incentive to book by phone!"
So it appears that you just cant change online you must call. Not sure why that is but you can change without the fee.