I would ring motability rather then email you’ll get a quicker outcome.
As said it shouldn’t be a problem for them to offer the new AP and yes you are free to cancel right up until you enter the PIN number on collection day.
But I can’t see that needing to happen, just tell them you know that you could.
If you did have to cancel and if it’s a standard car you’ll get your deposit back no problem.
it’s only if you ordered and paid for extras you may lose some or all.
ive had a similar experience in the past with Honda, I made the mistake of paying the AP up front. The small print said the same but after contacting motability I got my money back, so if you have any problems let motability know.