I hope JKR isn't going to get too precious over all this (as opposed to being disappointed about the disclosure). She's a novelist - a very successful genre novelist - who's written a novel in a different genre. Just as she did with her previous novel.
She isn't a Prime Minister found to be secretly writing porn, or a literary writer found to be writing in a hated genre ("No, it's speculative fiction" or "Yes, a crime is solved, but the book is a psycho-sociological examination of a very particularly milieu, a Cambridge college, not a sordid police procedural").
JKR's reputation will in no way be undermined, unless the book is poorly written compared to her more famous works (which, it seems, it isn't).