In this kind of fanfic, the development of characters would be fascinating. A Jedi character might start to develop a sense of superiority that is common among the Sith. They could become more secretive and less open to others. Meanwhile, a Sith character in the Jedi's shoes might initially try to resist the Jedi's peaceful ways but gradually find some value in them, leading to an internal conflict within the character.
The characters would develop in very unexpected ways. For instance, a Jedi who becomes a Sith would have to unlearn all the Jedi teachings. Their personality might change from calm and collected to more aggressive. On the other hand, a Sith in the Jedi's place would have to pretend to be good, which could be a real struggle for them.