Evolve Stage 2 is an overhaul of the game formerly known as Evolve. Turtle Rock, together with 2K, has completely remade the game. It is a new beast now. Apart from the free-to-play model, the asymmetric shooter received new game design, gameplay mechanics, and improved gameplay balance as well as system performance.
If you are not new to Evolve, probably, you are wondering what the changes mean to existing players. Chris Ashton and Phil Robb, co-founders of Turtle Rock Studios explain it all in a letter to the Evolve community, which can be read here. Every existing player, as well as everyone who will buy a copy of the game, will receive a founder status. More info on the topic can be found here.
Lastly, these two trailers are here to tell you the story of the changes: