Similarly, Rowling is the sole locus of information about the Harry Potter universe. It literally exists entirely in her head, so her word is law. She also knows she can always stay in the news by revealing some trivial bit of information about the characters or plot that wasn't in the books or films because fans will gobble it up like pumpkin pasties, but these revelations sometimes carry severe consequences for the story long after it has ended.
"Wizochoc is people!"
Her most significant retcon happened all the way back in 2007, when she announced that Dumbledore was gay. She certainly must have had noble intentions, making a beloved character part of a community that still faces a lot of discrimination, but it demonstrated her willingness to irrevocably alter the context of the story after the fact just for funsies. And when grilled by fans, Rowling will toss out tidbits on what everybody from the Dementors to the house elves are up to nowadays. More recently, Rowling decided that she'd made a grave mistake pairing Harry and Hermione with ginger members of the Weasley family instead of each other. It seems to be only a matter of time before we find ourselves in a "Snape shot first" situation, because ...