the more common deal is that a dealer will agree to cancel an order and reorder if the ap goes down. usually if someone is set on a particular model ordering at the end of a quarter you can run the risk of the ap changing. fine if it goes up your locked in at the lower rate by ordering before quarter end. not so good if it goes down.
so some dealers will adjust things so you pay the lower ap.
i havent come accross a dealer that would be happy for you to cancel and go elsewhere.