Cancelling an excursion and rebooking is easy on MVJ. Simply click on the reservation you want to cancel, (either from the calendar page or excursions page,) and click Cancel reservation. Then immediately book the one you want by putting it in your cart and paying just as you originally did. I’ve also done this over the phone, but most recently, the phone agent directed me to DIY. Constantly check for availability of your preferred excursion. You can sometimes find an opening onboard with guest services. Don’t give up!