We did that last November. We purchased annual passes ahead of time and had even used several days before buying
DVC. We took our temporary membership card and annual passes to a ticket window. ( I don't remember if it was a regular one or guest services-just ask!). They refunded us the discount (almost $400 for our 2 adult and 2 kid passes. Needless to say we were VERY excited as we didn't even plan on purchasing DVC. They just refunded the money back to the credit card I used to purchase the tickets.
Hope this helps!