see this link:
http://www.hackingblockbuster.com/20...exchanges.html
BB is testing a new program that will soon be rolled out everywhere:
"The final change is that Blockbuster is now counting the in-store exchange as one of the plan DVDs, and won't ship the next movie in a customer's queue until they return the in-store exchange."
I noticed BB stopped shipping new discs the day after I exchanged one in the store. To get around this, I reported the discs as already returned, but they might close this loophole.