I'm writing a book about a Human colony expedition that arrives on a planet and finds that it is populated by natives. Iron Age tech. The main character is the brother of the ships' captain (he has a boss, the owner of the expedition), and becomes an unofficial ambassador. After initial shock the humans hit it off well with the natives, they even sell the humans, but an incident triggers tension leading to the human leaders to start oppressing the natives. The captain is one of the ones who does this, and that is inevitable in the story. But for the first chapters of the book, already past 40k words, he is a very decent guy. That must change but I'm worried if it's too abrupt it will seem forced. Any tips on character changes of this sort would be appreciated.