What change do the choir boys undergo on the island?
The choir boys make two transitions in their character under the leadership of Jack Merridew. At first, the choir boys enter the scene as an older group, and they appeared still in their uniforms.
After time to establish roles and leadership personnel, we find the choir boys take on two responsibilities: tending the fire, and hunting for food. These are rather adult-like roles and they demonstrate the ability to tackle the bigger tasks.
However, after the fire goes out in chapter 4, we begin to see these boys undergo a great change. They rebel against the idea of hunting for the sake of responsibility. Their new purpose in the later chapters is to become savage and violent. It may be that this has occured and was not something they could control. But given the circumstances of having no one who could make them maintain order they turn primitive and even take life from another human.