How does Katniss Everdeen change in The Hunger Games? Please provide original (before) and change (after) quote and explain them.

There are two ways in which Katniss Everdeen changes as she reaches maturity in The Hunger Games. First of all, she undergoes the kinds of personal and emotional changes that other characters of her age might in a typical coming-of-age story. These may include asserting independence, a first love, or rebelling against authority. The second way has to do specifically with the particular circumstances that Katniss has to navigate. Because she is both fighting for her life and being manipulated for a larger purpose, she must learn not only to follow her gut instincts, remaining true to her tendency to be loyal and honest, but also to deal with the fact that she is being challenged by deceptive people around every corner.
Another aspect to consider is the way that her story mirrors that of the heroic journey, as set forth by Joseph Campbell. When Katniss volunteers as tribute for her younger sister, this can be seen as paralleling the call to adventure. She does this out of a desire to protect her sister and ensure her safety, but there is also a sense that Katniss seeks out this adventure, because she has been preparing for it by sharpening her skills at archery and survival.
But even though the adventure is a real-life physical one, there is also a great deal of artificiality surrounding it. Part of Katniss's growth and change encompasses learning this difficult truth and doing her best to expose it without endangering her own life or the lives of others. Katniss acts out of loyalty and a sense of fairness, and these qualities are on display for the world to see. This makes her a bit of a folk hero, someone fighting for the rights of the poor and the suffering. Katniss understands the significance of her role but finds it painful to cope with the realization that she is part of the system that rewards some people and punishes others simply by virtue of birth or geography. She sees inside the lives of those who live in the Capitol and is repulsed by the luxury and entitlement she sees there.

What must it be like, I wonder, to live in a world where food appears at the press of a button? How would I spend the hours I now commit to combing the woods for sustenance if it were so easy to come by?

Katniss sees her role as one of exposing the corruption and hypocrisy of the system, but she knows she must tread carefully, because even as her role as a hero makes her very valuable to those who control the Hunger Games, they could easily find a way to turn the public opinion against her. These very adult situations and her realizations about her place within them account for the many dramatic changes in Katniss's life in a short period of time.