The problem with books is that they are never finished, at least not from their author's point of view. I don't believe there's a writer – or any other artist – in the world who wouldn't like the opportunity to revisit an old work and improve it, because artists are a) perfectionists and b) exasperatingly self-critical. But with every book there comes a point (as my editor likes gently to remind me) when you have to draw a line, release your white-knuckle grip on the manuscript and let it take its chances in the world. And once it does that, it's no longer exclusively yours – the book and its characters belong in part to the readers.
I don't blame JK Rowling for having second thoughts about pairing up Ron with Hermione at the end of the Harry Potter series (I was never convinced by that relationship), but she should have kept those regrets to herself. The story is written and it feels like a betrayal of her characters to now undermine the decisions she made in writing it...