If the difficulty is going to be handled using the echo buff as everyone assumes then the content itself isn't being touched. Instead players just get a buff to make themselves stronger with every wipe. Old coil will still be old coil, it'll just have the lockout removed. We also don't know if a loot lockout ala CT will be imposed in it's place either.

Bear in mind also that SE seems to like having a lore-friendly reason for there being different difficulties of some instances. I can't see a reason why Coil would suddenly get easier and have two different versions of the place. The other HMs actually take place after the normal modes...