Personally it has never happened to me that the ap has gone down after ordfeing. but it make’s no difference to the dealer what the ap is, Only they have to cancel the order and re-order at the new ap (which is more work for them) but seeing as your exchange date has been extended, i don’t see it being an issue at all.
You deffo need to speak to the dealer as they the only ones that can order cars.
The dealer takes the ap on the behalf of mobility, even if they offer you a deal on the ap they still have to give the full ap to Mb that it was in the quarter ordered. so cancelling and reordering is deffo the way to go as £599 is quite abit of a saving.. Once you go get the £600 gcb from mb then you’ll be £200 up rather than £399 out of pocket.