Of course the AP 2 will be compatible, they just won't have whatever new features are in the new Pencil.
What makes you so sure?
The Apple Pencil 1st gen does not work on any of the iPads that were made for the Apple Pencil 2nd gen. If you got a new iPad, you had to get a new pencil too. Sure, there are ways/hacks to get the AP1 to work with the newer iPads, but they aren't officially supported.
Also, the new M3 iPads are rumored to have a landscape camera, which would be in the way for the magnetic/wireless charging module that the AP2 uses (which is presumably why the 10th gen iPad didn't get AP2 support). We don't know how the new pencil is going to magnetically attach and charge. If it ends up getting a different style magnetic/wireless charging module, it may not be backwards compatible with the AP2. If that's the case, how are you going to pair the AP2 and keep it charged? At least with the AP1, one could still use a cable to pair and charge it if they managed to get it working with a USB-C iPad.