How do I start using the new app? I presume I have to convert my account?
You don’t start using it until Samsung is ready to convert everyone over to it.
If you are running the SmartThings Classic app now, then that’s all you need. Leave the new app alone. You don’t need to install it.
You don’t, until you hear from support your account is ready.
To do otherwise is to invite all manner of gremlins in to screw stuff up.