A perfectly reasonable question and the answer is ‘yes’.
If your order a car and the AP drops before delivery, the dealer can adjust it for you, but check before you order that they are happy to do this (most are but some can be ar@#holes 😂).
If a car that you prefer to the one that you order comes on the scheme, simply cancel and order the new one.
It’s better to do the above (assuming that there’s a car on the scheme at present that you want) rather than hold off in the hope that APs will drop. The trend in APs has been upwards for around 2 years now and I don’t see that changing. If the APs go up you are protected, but if they drop you can do as suggested above, so there’s nothing to lose. If, however, you hold off and APs rise, you have absolutely no choice but to pay the higher price.