I understand your feeling of misfortune due to the AP changing for a lower rate, this has just happened to our vehicle which was ordered last week 23/03/23 and has since had the AP reduced from £7999 to £5999.
because the AP was agreed before the April update it remains at the higher rate and this is in the terms and conditions on the MB web page, i just wonder how the dealer feels about this situation as i am not best pleased to be paying a considerable amount more for such a short period of time.