This conversation has devolved a bit, but I wanted to throw out my own opinion in on the original subject.
The Hermione Granger in the books was white. Now if they want to change the character and say that she is now black, that's fine. Just admit it is a change.
Nick Fury was changed in the Marvel movies, but everyone recognizes it was a change from the source material. Likewise with Ford Prefect in the Hitchhiker's Guide to the Galaxy. Really the race of either character doesn't matter, so changing it is no big deal. It is also not a big deal changing Hermione, as it really doesn't matter to the story.
But as far as casting goes it still matters because Hermione's parents are going to have to be black as well. Otherwise it wouldn't make sense. Also one will need to be male and one female. Unless they also want to change it so that she has two mothers or fathers and she was adopted or was conceived invitro or... Well, it starts to get ridiculous. Just cast a black mom and dad and move on.
So no, you can't just "pick the best actor". I mean, Richard Harris is a good actor, but you aren't going to cast him as the red-headed 9-year-old girl Ginny Weasley. He made an excellent Dumbledore though.
One final note, Hermione was discriminated against because of her parentage. She was born from Muggle parents. In Muggle society she was a fairly well off (parents were both dentists) middle-class white girl. So she went from a society where she was part of the privileged race, to one where she was the oppressed minority. Whereas a black Hermione would go from one form of discrimination to another (racial and then magical). Still an interesting idea to explore, but different than the book.
Oh, and I have read all the books and watched all the movies.