In Lord of the Flies, why does Jack's name change?

Throughout the novel, Jack's character develops from a boy who has feelings of grandeur to a leader of a band of vicious hunters.

Jack is desperate to be leader and exert his authority from the moment he is introduced in the text. Because of his ability to sing a "high c" and probably due to his age, he is leader of the choirboys. Although it is extremely hot on the island and the boys have just survived a plane crash, Jack has the choirboys wear their robes. This act distinguishes the choirboys from the others and, in a sense, elevates them (and by default Jack).

Although the boys do not elect Jack as the "leader," Jack insists on some type of leadership role. He asks that Ralph name him leader of the hunters. Initially, this idea is more about fun and games than actually tracking and killing. However, as the boys remain on the island and detached from civilization, the hunting aids in developing Jack's predatory and leadership skills that are exemplified earlier in the text. As time goes on, Jack is less afraid to kill the pig and becomes determined to kill the pig with his homemade spear. The killing of the pig symbolizes Jack's ability to lead, to provide, and to be superior to Ralph. 

Once the boys, a.k.a. the hunters, successfully kill a pig on the island, they have  bloodlust. When they return to Ralph and the others with the pig, Jack speaks first. He announces, "'I cut the pig's throat,' said Jack, proudly, and yet twitched as he said it" (97). This indicates that he is not comfortable with taking a life, but he is more amenable to the power that killing the pig affords him.

As time goes on, Jack's desire to hunt and dominate grows. He uses his ability to hunt and provide food to manipulate the others on the island. His argument: Jack can provide them with food and help them survive. What can Ralph provide them? Therefore he takes his followers (any boy who would rather play and hunt than work to survive) and sets up camp on Castle Rock. This is where Jack transitions from Jack to "Chief."

When he is designated as "Chief," Jack's persona changes. He is no longer afraid and more domineering. He allows other boys to be tied up and tortured. They fantasize about recreating pig hunts and using a littleun as a stand in for the pig. He steals Piggy's specs in order to control the fire, and ultimately wants to eliminate Ralph from the island. 

The transition from Jack to Chief is symbolic of the change in his character. It also reflects the changes on the island: loss of Simon, loss of Piggy, destruction of the environment, and a loss of civilization.