I'm currently using a few wAP AC (old ones) mounted on the wall about 20cm from the ceiling. I'm thinking of replacing them with the new cAP AX. As the old ones are wall mounted access points and the cap is a ceiling mounted access point I'm wondering how this will affect my coverage when using the same mounting location on the wall. Anyone have any idea?

Should not be a problem given cap AX has much better wireless capabilities than wap.

Would the the difference with a hAP AX be significant? (It's not an option as it doesn't support 48v PoE, but curiosity want to know!).

As replacement for wap, probably not.

So I do wish MikroTik could develop a model which use the WAP series case and new AX motherboard.
Let the user who use old wAP models can upgrade to AX model more conveniently.

I'm waiting for that option too.
Since I am not sure mANTBox ax 15s is the correct replacement for wAP.