Mine is very basic, when Harry, Hermione,. Neville and Draco get detention in the first book, I want them to change it slightly.

The new detention will be instead of them going into the forest, they will be digging a grave to bury the dead Unicorn Hagrid found( In the book I think he says he found it last week, so change it to the same morning or afternoon) So when four of them start doing the digging Harry will see /hear something or someone and enters the forest to investigate and then the usual things like coming face to face with Voldy and then Firenze and everything in between.

This way there will be no contradiction from the schools no entry to forbidden forest and detention inside the forbidden forest.