you need to get the dealer to agree to that before you order its not automatic as far as i know.
all motability guarentee is that the ap when you order is what you pay even if it goes up before you collect.
but most dealers will in effect cancel your order and redo it at the new lower price if the ap goes down.
if theres a quarter change due and you want to get your order in before the change i would get the dealer to confirm in writing that they will reduce the ap if the ap goes down in the meantime.