As noted, this is a sketch by JK Rowling of the main characters from "Harry Potter." This is how she saw them. It's fine that she's open and welcoming to the new interpretation of the characters in "Harry Potter and the Cursed Child," but it absolutely _is_ a new interpretation, and the fact is that, when she created the characters and throughout the writing and release of the novels, she intended the character of Hermione Granger to be Caucasian. So it's a little dishonest to be coy about it now. It's a change. It's a new version. Like it or don't, welcome it or don't, it's wrong to pretend it isn't.
There are indeed plenty of racists who object to changing the race of canonically-Caucasian characters, but it's not inherently racist. There are perfectly legitimate reasons to find it inappropriate. Put very simply, white people have a very different experience of life than people of other ethnicities, and that affects everything about who all of us are as people. White people like me live in impenetrable bubbles of privilege, and everything we experience and are formed by is defined by that.
A Hermione-centric example. Being called the "M-Word" is going to mean something very different to a twelve-year-old white girl who has grown up in that bubble of privilege than to a twelve-year-old black girl who has lived in the shadow of the "N-word." How she'll feel about and react to that kind of bigotry will be vastly different.
Now, if you think the practice colloquially called "race-bending" is a fine way to make media and entertainment more inclusive, that's fine, and I can respect that, even though I disagree with you.
You owe people like me, who think it disrespects all sides of the equation by erasing the differences that various issues around ethnicity and privilege make in everyone's life and character, equal respect. It's not racist to have this objection.