neil, if your car hasnt been delivered yet the dealer can simply cancel the order and reinput it at the lower ap. its just a paper shuffling exercise doesnt affect your place in the queue so to speak.
i could do the same as the ap on the nissan juke i have ordered has gone own from 631 to 90 but the weekly rental has gone up so its not worth me changing as it would actually cost me 500 extra over the 3 yrs.