The other students at Hogwarts would be impacted too. Harry has been a symbol of hope and resistance against the Dark Arts for many of them. If he stops caring, it could cause a wave of despair or confusion among the student body. Some might even lose their own motivation to fight against the growing darkness in the wizarding world.
Dumbledore would also be affected. As a mentor - like figure to Harry, he has certain expectations and plans for Harry. If Harry stops caring, Dumbledore's grand schemes against Voldemort might be derailed. He would have to find a new way to motivate Harry or adjust his strategies.
Maybe Harry has faced so many traumas and disappointments in the wizarding world that he just decides to distance himself from all the chaos. For example, he might be tired of the constant battles against the Dark Arts and the politics within Hogwarts and the Ministry. He could stop caring about his reputation as 'the Chosen One' and simply want to live a quiet life, away from the spotlight and the expectations.
Well, in many Harry Potter fanfics, even those named 'the change', the main characters often include Harry, Hermione, and Ron. However, depending on what the 'change' is about, new characters could be introduced or existing minor characters could be promoted to main character status. For example, if the change is about the Slytherin house becoming more inclusive, we might see Draco Malfoy taking on a more central role in the story, along with the usual trio.
Uma would be one of the characters affected. Since she was often associated with Harry Hook when he was 'bad', his change might make her reevaluate her own actions and beliefs. She could either decide to follow his lead and also turn towards good or struggle with the change in their relationship dynamic.
One possible reason could be that he has been through so much in his original adventures that he's just tired. He might feel like he's constantly being put in new, often contrived, situations in the fanfiction and he simply wants to step away from all the chaos and drama.
Well, it could start with Harry gradually noticing how the fanfiction world is not in line with his own values or experiences. For instance, he might find that the magic system is misrepresented in a way that he can't accept.
It could also focus on some of the minor characters from the original series. For example, Neville could be a main character in this fanfic and his growth and change during the summer could be the main focus. Or Luna, with her unique perspective, could be in the center of all the happenings during that summer of change.
Hermione would surely be one. She's smart and would be great at helping with the academic side of building a school, like creating the curriculum.
Ron Weasley could also be a main character. He might be dragged into some adventure in the Prefects Bathroom by Hermione or Harry. For example, he could accidentally set off a magical trap in the bathroom and they all have to figure out how to get out of it without getting into trouble with the teachers.
Well, it would change the power dynamic. Usually, Dumbledore is seen as a guiding figure for Harry. If Harry stops listening, he might make more mistakes as he lacks Dumbledore's wisdom. For example, in battles, he might not make the strategic decisions that Dumbledore would have advised.
His friends would surely be affected. Hermione and Ron might be both amazed and a bit worried. They could be worried that this new power might change Harry too much or put him in more danger. Also, his enemies like Draco Malfoy would be affected. Draco might become more jealous or even more afraid of Harry.