Where in Lord of the Flies does it show that Jack has replaced his need for becoming the leader of the group with being head chief?
In Lord of the Flies, Jack reveals his ambition to be chief of the tribe from the very beginning. At the first assembly meeting, Jack insists he should be chief because he is "chapter chorister and head boy" (22). As the story progresses, Jack's determination to be chief leads him to undermine and challenge Ralph's authority:
"'[Ralph's] like Piggy. He says things like Piggy. He isn't a proper chief'" (126).
In chapter eight, "Gift for the Darkness," Jack splits from the tribe, calling out Ralph as a coward, citing his lack of experience in leadership as the main reason he should not be chief. Despite Jack's protests, none of the other boys publicly disavow Ralph as chief, choosing to slip away and join Jack's tribe without drawing attention to themselves. Jack uses his hunting prowess to draw the allegiance of the boys:
"Far along the beach, Jack was standing before a small group of boys. He was looking brilliantly happy.
'Hunting,' he said. He sized them up. [...] 'We'll hunt. I'm going to be chief'" (133).
Jack connects his ability to hunt with his natural predilection for leadership; in his mind, the best hunter should be chief. He uses this rationale to justify his actions in taking control of the tribe.