If it is 31 days or more before your existing reservation date, simply call Member Services or visit DVCMember.com. On the phone, ask the operator to make the changes. On DVCMember.com, go to the "My Membership" menu, find your reservation and make the necessary changes. If your new reservation uses fewer points than the previous one, the leftover points will be restored to your account.
If it is between 1 and 30 days prior to the reservation date, use the same process to change it. However, any leftover points will be transferred to a holding account --- which means they can be used only for reservations made up to 60 days ahead of time.
On check-in day, you can use the same processes or change your reservation at the resort itself. However, any leftover points will be forfeited.
