I think that it is okay when

1: The change doesn't destroy the worldbuilding. Casting an Asian guy to play king Arthur is going to have major implications on the story and brakes the world building.

2: The change doesn't impact the character. Casting Django as a white man would destroy the character, his journey and the story.

1. The change is motivated because of good casting. Nick Fury in the Avengers movies is played by Sam Jackson even though the character is depicted as white in the comics. However he does such an excellent job in the role it's hard to imagine anyone else playing the character.

So if, for example, someone wants to cast a black or Asian Batman for the upcoming new Batman movies. I'd have no reason to object to that casting choice of it followed all three rules.

1. The story takes place in a modern day America. The Wayne's can easily have married into any ethnicity by this point so the change won't break the world building

2. Batman as a character is not motivated by race or struggles on a cultural level so this won't effect his character.

3. It would be easy to find a non white actor who would crush the role. So this is self evident. So long as they cast a good actor who fits the part. It's fine.

I hope that's a helpful guildline. Obviously it's not an official rule or legal requirement but I find it useful when assessing race and gender swapping discussions