Removing a device or two shouldn’t “destroy the mesh.” I did it moving from ZHA to Z2M.
You want to migrate logically enough to keep a functional mesh for both networks. There might be a brief window with less than ideal redundancy, but chances it shouldn’t too difficult to keep things functional for both networks.
Even if you have to buy an extra plug or two to bridge a gap, it is worth it to have the ability to migrate at your leisure vs trying to do everything overnight.