To write a good sudden - change HP fan fiction, you need to start with a strong concept for the change. It could be something as big as a change in the magic system itself. Once you have that, you should introduce the change gradually. Don't dump all the information about the change at once. Let the readers discover it along with the characters. Also, consider the consequences of the change. If a spell that has always been harmless suddenly becomes dangerous, how does that affect the characters' daily lives? How do they adapt? And don't forget to add some twists and turns to keep the story interesting.
First, focus on the characters. When creating a sudden change, think about how your favorite HP characters would realistically react. For example, Hermione would likely immediately start researching to understand the change. By staying true to the characters' personalities, your fan fiction will be more engaging. Second, build a solid plot around the sudden change. Don't just have the change happen for no reason. Maybe it's a result of a dark magic experiment gone wrong or a prophecy coming to fruition. Finally, add details about the HP world. Describe how Hogwarts or Diagon Alley looks different because of the change. This will make your fan fiction more immersive.
Begin by choosing the type of sudden change you want to write about. Is it a change in a character's appearance, like Ron suddenly turning into a werewolf? Or is it a change in the political situation of the wizarding world? After that, create a believable backstory for the change. If it's a character - related change, perhaps there was a family curse that was dormant until now. In your writing, use vivid descriptions of the HP universe. For example, if there's a sudden change in the Forbidden Forest, describe the new - looking trees, the strange creatures that now inhabit it. Also, make sure your characters grow and develop because of the sudden change. They should learn from it and become different in some way by the end of the story.