I would say it depends on the audience and what they're basing their experiences on. I'm sure there are differences in opinion of those only familiar with the books, those only familiar with the movies and those who are not familiar with any of the previous resources.
For example this: I have absolutely no idea who Miles Morales is, so I have no judgment on this character at all. People can only judge based on their own experiences.
In the case of Hermione, I am judging everything based on my own experiences from watching the movies and not reading the books, so I can only compare future endevours to the movies, such as changing her from white to black, regardless of what she might have been from the movie. It doesn't mean a person's view based on the books are any less important than my views based on the movie, however, just different sources.
Would it be a factor of change-concern if this was a movie intended to be "year 8" of the Harry Potter series, and they just swapped in a black actress?
Or if 10 years from now, they do the remakes of the HP series, and recast it to better match the UK's demographic?
Or if this is somewhat separated new HP story, from a different studio, who wants to do their own interpretation (which is kind of what this play is)?
Personally, I think I can roll with it. obviously, I'll have my silly "hey that's not how I think it should work" moment like we have here, but I'd probably go see it, and if it's decently done, be happy with it.