@Awas, the short answer is I do not know.
The long answer is that the new App will work with existing device. In the future (not at this migration stage), the custom driver may not work anymore. I am not 100% sure for Zigbee device. The child device driver for expansion may still work at this coming classic to new app migration. There may be some small changes on DTH meta data is needed.
The future ST migration stage probably is more challenging for us. The classic deprecation in theory should still support us.
Obviously, I am not an expert on the classic to new app details. I could probably wrong.
I will try to keep up with ST if the conversion do not require significant effort.