i thought HMRC can back date it and go after the contractor especially if they have left. But I think you are right HMRC is shifting their focus to companies as opposed to individuals to claw back tax liabilities.
If a contractor was claiming to be working outside IR35 and then migrates to the new rules and the client issues them their SDS which deems them to be inside IR35, then if that contractor then continues inside IR35 then yes HMRC could argue that nothing has really changed and the contractor could have been inside all along. HMRC could then pursue the contractor for back taxes. HMRC has stated that they wont do so up front but who on earth believes that!