Seem to recall that it wasn't actually retconning, though: the screen adaptations were going to depict Dumbledore as having once been in love with a woman, so Rowling just told them that wasn't possible, because he's gay. Which then got made public. But it sounds as though she was simply doing what the studio had hired her to do: keep the films at least reasonably faithful to the books. By not straightwashing a gay character, for example. And that seems perfectly fine to me. (Full disclosure: have never been much of a Potterhead myself.)