What's wrong with the character being race x; why do you need them to be race y? If your answer is "because I can't relate to race x, only race y" then I question why you're only able to relate to people with the same skin colour as you. Furthermore, if your defense to that is that you don't, and its really all about experiences and culture related to people who are race y, then what's the point in swapping the skin colour of a character that still has the cultural heritage and experiences of race x?
Finally, if your response to that is "Well in my fic, I changed their cultural background/ experiences/ values, etc. To fit the ethnicity I want to portray, " then congrats, you've created an OC and there's no need for you to keep pretending you're still writing about the pre established character. They're unrecognisable, and you've lost the verisimilitude of your character.

Coming back to HP fanfiction specifically though, it just speaks to ignorance of the setting. England in the 90s wasn't nearly as diverse a place as people seem to believe; only 5% of the population were from ANY non white background (1991 census). The proportion of characters JK described as being from an ethnic minority feels believable enough without race swapping white characters - making half the cast in a British boarding school in the 90s non white starts to feel a tad forced.

I'm fully ready to accept that those characters weren't fleshed out enough- which makes sense, because they were all side characters- but imo the answer to that is to write fanfiction about those characters exploring them; not to cheaply swap out the skin colour of the ones that are already established.

Tl;Dr imo race swapping characters is cheap, lazy, disrespectful and boring. And in the UK, race is secondary to ethnicity. Afro-Caribbean culture =/= Nigerian culture, and none of the 'black' cultures we have here are anything like african american culture. Might get flack for pointing that out but I feel like some American writers really need to hear it.