I’m not sure this is quite cut and dried as you’d hope, but the first thing that comes to mind is Lavender Brown’s skin colour.
(In)famously, by now, Lavender Brown was a minor character in the first movies, with no lines. She was portrayed by two black actresses: Kathleen Cauley (unverified) in Chamber of Secrets and Jennifer Smith in Prisoner of Azkaban.
When the first movies were made, of course, Half-Blood Prince wasn’t out yet and the casters didn’t know that Lavender would become an important character later on. The books never actually give a proper description of Lavender, but Half-Blood Prince does have the following quote:
Harry looked into the corner she was indicating. There, in full view of the whole room, stood Ron wrapped so closely around Lavender Brown it was hard to tell whose hands were whose.
While this could just be hyperbole, it at least implies that Lavender is white. If she were black (at least if she were as dark in skin tone as Kathleen Cauley and Jennifer Smith are), there would be absolutely no trouble telling her and Ron’s hands apart, regardless of how closely wrapped around each other they were.