Jurisdictions with a 70 year post mortem period are welcoming into the public domain this year works from people who died in 1937 – people such as children’s author Jean de Brunhoff (Barbar the elephant); Author Sir James Matthew Barrie (eg. Peter Pan); Musician and composer George Gershwin (eg. Rahpsody in Blue). (Thanks to WIPO for this list).
Each time something (almost anything) is reduced to material form – copyright is created in it. There is no concept of ‘novelty’ or ‘newness’ (or in many situations even creativity) necessary for copyright protection.
So to ‘evergreen’ a copyrighted work – just keep reproducing it in slight variations, each of which will have its own, new copyright term. Sure, competitors will be able to copy the original work once the post mortem period has ended, but the public (and your market) will have a stronger relationship with the newer versions, not the old ones.