This one takes a little explaining.
Firstly, a game keeper is not someone who looks after a game like football; he looks after animals that are hunted for food and also sport. Game birds are particular species of birds such as pheasant and grouse which are bred, sadly for them, to be shot as a game.
Even now, particularly in Scotland, people go on shooting holidays on the large, country estates.
Traditionally, these were owned by a Lord or Laird who employed a game keeper to manage the breeding of the birds, the shoots and to protect the birds from locals hunting them for their own needs. In the past, the methods of protection could be brutal such as iron leg traps hidden in the forest and the punishment from the local magistrate, who was probably related to the lord, would be severe.
So who is the poacher? The poacher is the one who steels and hunts on the estate.
When someone changes their role from the person steeling to the person protecting, we can say that they are poacher turned game keeper. This could be a computer hacker getting a job protecting systems.
One of the most famous examples of this was the real life confidence trickster (con man), and fraudster, Frank Abagnale, whose life was portrayed in the movie, Catch me if you can.
Well, the FBI did eventually catch him but then they employed him! He was a real poacher turned game keeper.