The real issue is whether the Senate will be reorganized with a clause allowing republicans to take control if the membership shifts. Republicans need to insist on it. Of course they will be called heartless, etc. I predict they don't do it, meaning we could control the Senate and not control the committees, etc. - meaning we wouldn't control the Senate.
The Pubs better not wimp out like that if this occurs. As I recall, the Dems didn't cut any slack when Jim Jeffords switched parties and turned the balance of power in the Senate.
The Republicans can take control the moment they become the majority party. If Johnson never sets foot on the floor of the senate for the remainder of his term there are still 50 Democrats and only 49 Republicans.