One event could be the loss of a very important person in his life. This loss could shatter his previous beliefs and make him think that being soft and warm - hearted was a weakness. He might also have been exposed to a group of people who took advantage of his kindness. As a result, he decided to toughen up and become cold. Also, perhaps he went through a series of difficult battles where his lack of strength nearly cost him his life, so he trained hard to become strong and in the process, became cold as a defense mechanism.
His relationships would change a great deal. The friends who once knew Harry as a somewhat vulnerable and kind - hearted person would now have to deal with this new, cold - edged strength. Those friends who were closer to him for his softer side might drift away. However, some might see this as a positive change in a way, like they might think he's finally growing up and becoming more self - reliant. But still, there would be a lot of adjusting to do on both sides.
One possible plotline could be that after facing a great betrayal, Harry decides to close himself off emotionally and focus solely on becoming stronger in magic. He might train in isolation, away from his friends, and develop new and powerful spells.
The Ministry of Magic could turn into an antagonist. If Harry's new cold and strong ways are seen as a threat or too radical, they might try to stop him. This could lead to a conflict between Harry and the wizarding government.
One possible reason could be a major betrayal. Maybe someone he trusted deeply, like a close friend or a mentor, turned against him. This would make him lose faith in people and become cold as a form of self - protection.
One possible reason could be a great betrayal. Maybe he discovers that someone he deeply trusted, like a close friend or a respected mentor, has been using him all along. This would shatter his trust and make him withdraw from people, thus becoming cold.
One possible event could be a false accusation within the wizarding community. Maybe someone frames Harry for a serious crime, and his so - called friends believe it without really giving him a chance to defend himself. Another event could be a power struggle within Hogwarts or the wizarding world in general, where people see Harry as a threat and turn against him to gain an advantage.
His life at Hogwarts would be very different. He could fly around the castle freely in his phoenix form, which would be quite a novelty. He might use his form to play pranks on the Slytherins in a way that they couldn't easily catch him. Also, his classes would take on a new meaning. For example, in Transfiguration, he could study his own form more closely and learn new things about Animagus transformation. And he would probably be the center of attention among the students, with everyone wanting to see his phoenix form up close.
Perhaps in the fanfiction, his friends failed to recognize his efforts. Harry might have worked really hard on something, like preparing for a big event or helping them in some way, but his friends didn't seem to appreciate it at all. This lack of recognition could easily make him disappointed in them.
Perhaps he witnesses a great evil that he could have prevented if he had been more accepting of his role earlier. This guilt and the knowledge of the consequences of his inaction could drive him to accept his destiny. Or, it could be that his friends are in great danger because of his hesitation, and he decides to embrace his destiny to save them.
The characters around Harry would likely change. Some minor characters might become more important, and some major characters could be sidelined. For instance, Neville Longbottom could become the main hero instead of Harry in some scenarios.