They are people willing to make moves. That's basically Jeff's definition of a game changer. Sarah was willing to vote out her original tribe, Troyzan was the only one who went after Kim/the girl's alliance, and Michaela basically had the entire MvGX season planned out how the boot order would go.
Sarah was picked because the Queen of Cagayan Chaos Kass voted her out and Kass's move change the game. It was Sarah's blindside that changed the course of Cagayan. Sarah didn't change the game but Kass did by blindsiding her and therefore Sarah's vote out is considered a "Game Changer" not necessarily her as a player but the way she went out.
Sarah is just a good strategist not a "game changer" idk about Troyzan