I can't believe no one has replied to this. I recently purchased a left handed CZ457 Varmint.
The barrel swap is very easy, remove two action screws, remove the action and barrel then loosen the two grub screws on the barrel to swap the barrel. Easy Peasy.
As for threaded the Varmint barrel is not threaded but it can be easily threaded by a gunsmith mine charged $95.
I believe the American barrel may come threaded if not after market or older threaded barrels are available.