This thread is referring to migrating from the Samsung smartthings V1 hub to the Samsung smartthings V2 hub. Not migrating from the smartthings classic app to the new smartthings app.
As of this writing, March 13, 2018, support has advised everyone currently on the classic version of the app to stay there as the migration work is not finished and if you do try to use the new smartthings app you will lose some functionality. So just stay put on the smartthings classic app until you receive an individual notification from support that they are ready to migrate your account.