When the UK changed the time for copyright from 50 years after the author's death to 75 years a lot of works that had been out of copyright, including Kipling, went back in again, so yes, it is possible to 're-copyright' (and, of course, the copyright of Peter Pan was regularly 'updated' by separate Acts of Parliament and now belongs to GOSH in perpetuity - at least in the UK).

Gatiss and Moffat have the rights on their scripts for Sherlock, and the BBC has the rights to the actual series, but of course there is not (and never has been) 'copyright on ideas'. I am getting fed up of pointing out that you can only infringe 'copyright' by making an exact (or substantially close) copy of a work - that's what 'copyright' means!