Doon changes more than any other character in The City of Ember. At the beginning of the story, he is committed to saving Ember; he is quiet, moody, and more inclined toward mechanics than people. By the end, he is more confident and willing to look toward a life of uncertainty.
When Doon gets the job of Messenger after graduation, he gladly switches with Lina. His dream is to fix the generator and ensure that Ember continues to get power. He quickly realizes that this isn't the way to save the city. Later, when he's still trying to find ways to solve problems for Ember, he's classified as a threat to the city.
This classification is the one thing that really changes Doon. It forces him to leave the city along with Lina and Poppy. If their news and findings had been met with positivity and acceptance, he likely would have stayed in Ember. Instead, he's forced to leave and seek a life outside the city that sheltered them. His curiosity and desire to make the community better are punished, and this must disillusion him somewhat as well.
In the end, though, Doon still tries to give back to Ember. Though he, Lina, and Poppy can't go back, he tries to find a way to communicate what he's learned to the rest of the people there. Though he's changed in that he was willing to leave and open up more to Lina and Poppy, he is still a caring person and wants to make sure that others in Ember aren't left in the darkness.