How does Jerry, as the main character, change from the beginning of the book to the end? What causes this change?
Jerry seems to be a traditional hero from the beginning of the novel. After being insulted by the coach on the football field, Jerry is determined to prove him wrong and make the team. The intention is for the reader to identify with Jerry, to root for him. Thus he's presented as one who will overcome his challenges and achieve his goals.
This carries through the entire "Chocolate War." Jerry carries on his boycott, with little reason. His best excuse is his poster, which quotes T.S. Eliot in asking "Do I dare disturb the universe?" It's Jerry's question for himself: Do I dare go out on limb? Go against the grain? Choose any saying you like; it's Jerry's attempt to establish himself as an individual. Whereas before he refrained from speaking when insulted, he is now standing up for himself and standing apart from the crowd.
However, it doesn't end well for Jerry. His brutal beating at the hands of Emile results in his whispered warnings to the Goober-Don't disturb the universe. The fact that he doesn't overtly succeed or win is what made this book so shocking for its time. It's often been banned, and one of the reasons is the perceived triumph of evil over good, as reflected in Jerry's beating.