Obviously, Harry Potter himself is the central character in this type of fanfiction. Then there are often characters like Voldemort or his Death Eaters, as they are the ones Harry would be changing sides to. Bellatrix Lestrange could be a key figure in trying to persuade Harry or manipulate him into joining their side.
The Death Eaters themselves would be affected in an interesting way. Some might welcome Harry with open arms, but others might be suspicious of his sudden change of sides. They might fear that it's a trick by the Order. And within Hogwarts, the students would be in turmoil. Those who idolized Harry would be heartbroken and confused, and there could be a split among the student body depending on who believes Harry's new stance.
Perhaps Harry discovers some long - hidden family secrets that make him believe that the so - called 'good side' has been wrong all along. He might find out that his family has a connection to the Dark Arts that was misrepresented by Dumbledore and others. This newfound knowledge could cause him to change sides and try to rewrite the wizarding world's view of good and evil.
Another plot could involve Harry being influenced by a family connection he discovers to a Death Eater. This newfound relation makes him reevaluate his beliefs and he starts to see the Death Eaters as more than just pure evil. He might think that by joining them, he can reform them from the inside.
One interesting idea could be exploring the complex relationships Harry has to re - evaluate. For example, his friendships might be strained as he changes sides. Another aspect could be how his view of the magical world's power structures alters. He might start to see the other side's motives in a different light, which could lead to some unexpected alliances.
The character development may start with Harry's doubts. He begins to question the side he was on, which leads to his decision to change. As he moves to the other side, he has to adapt to new values and beliefs. This forces him to grow and re - evaluate his past actions.
One good small change could be in the character development. For example, Neville could start showing his courage a bit earlier in the fanfiction. This would add an interesting twist as it would make the story deviate slightly from the original in a positive way. It could also change the dynamics within the group of friends at Hogwarts.
In fanfiction, 'Harry Potter changes Westeros' could be through his magic skills. For example, his spells like Wingardium Leviosa could be used to move heavy objects in Westeros, which might change the way battles are fought or construction is done. His knowledge of the magical world could also introduce new concepts to the people of Westeros.
Perhaps in this fanfiction, Harry discovers some dark secret about the Order of the Phoenix or Dumbledore that makes him lose faith in the side he has always been on. This, combined with Voldemort's manipulation, could lead him to side with the Dark Lord. For example, if he finds out that Dumbledore has been using him all along without his full knowledge, he might be so disillusioned that he turns to Voldemort as an alternative.
Obviously, Harry Potter himself is the central character as the title implies. Voldemort is also often involved as the main figure on the 'dark side' that Harry might switch to or interact with differently. Then there's Snape. His complex relationship with Harry could be a key factor in the story. In some fanfictions, he might be the one who either tempts Harry to switch sides or tries to prevent it depending on his own motives.