You can by installing the driver, removing the device from Smartthings and then reinstalling the device. You will loose all automations unless you use a virtual device as a place holder.
Or you can wait and see what happens after the transition.
It will go away
You can access devices through the CLI. There are some very good write ups with instructions how to do this. CLI: How to install and use
The app will stay.
Alexa and Google are both working with the new architecture so you should be fine. The only issue is that multicomponent devices that were installed with separate child devices on a DTH are now installed as a single device. Voice assistance can only see the main switch so you would need to mirror any additional switches to a virtual device to allow voice control of all the switches.