I was a bit surprised to see it suggested that a device with a custom device handler could not be paired with the new app, given that the new environment still uses Groovy based device handlers for Zigbee and Z-Wave devices and creating a new one still involves the IDE. So, out of curiosity, I deleted a spare Aqara vibration sensor from the system and attempted to pair it using the new app (I don't have the Classic app installed). I succeeded. It was a bit of a battle, but by no means the longest battle I have had with the Aqara sensors, not by a number of hours. It installed itself with the correct device handler. So, on the whole, I am fairly relaxed about the situation.