Sometimes fans get a little carried away and take ownership of these franchises because they love them so much. If JKR adds more stuff to the Potter-verse then she is then only one who can decide what is and isn't canon. She is the creator, what she says goes. It's like George Lucas and Star Wars.
I'm still trying to figure out where I come down on that whole discussion, to be honest.
When she said in an interview that she'd always thought of Dumbledore as gay, I pondered it, and thought, "Well, okay, I can see that. Doesn't really change how I read the books, but I can see how that would work, and it does make some things make a LOT more sense. Okay, accepted as personal head-canon."
When the big kerfuffle arose because they cast a black woman to play Hermione in "Harry Potter and the Cursed Child," I really couldn't see the reason to get upset, even though my personal image was a (much less attractive) version of what we saw in the movies. I know some folks point to the text in Prisoner of Azkaban where she and Harry are rescuing Buckbeak right outside of Hagrid's hut (with their "earlier" selves inside the hut under Harry's Invisibility Cloak) and it says, "Hermione's white face was sticking out from behind a tree," and claim that she MUST be white, but frankly, I can see that as merely an expression rather than as a prescriptive description of the character. Other than that one mention, there is nothing (that I remember) in the text that demands that she be any particular ethnicity, so mixing it up a little bit for a stage performance causes me no grief. I feel a little bit smug in having some validation for my personal head-canon, but if they want a non-white actress to play Hermione in a play, so what? Anyone who isn't Emma Watson is going to be jarring for me, anyway, now that I've seen her as Hermione so many times.
When she said that Ron & Hermione probably needed marital counseling, I thought that was just common sense. Heck, I figure several of the couples might have needed it. You KNOW Harry needed some counseling to deal with his issues, and Ginny had been possessed by Moldy Voldy, so HER head wasn't exactly on perfectly, so it stands to reason that the two of them as a couple just might have gone to seek counseling, too. I didn't take Queen Jo's comment about Ron and Hermione needing counseling (or her comment in the same interview that in some ways, Harry and Hermione were a better fit) as her saying that what was in the Epilogue was wrong, or that the Harmony shippers had been right all along; I took it as some common-sense observations. The Harmony shippers were wrong, but they weren't basing their hope off absolutely nothing-- there WAS something special between Hermione and Harry; it just wasn't the kind of special that leads to marriage.
However, when I read Cursed Child, and found out she thought that the plot was just peachy-keen and could be regarded as canon, I said, "Nope! Not canon for me!" When she removed (or allowed to be removed) the original birthdate for Minerva McGonagall from Pottermore that CLEARLY showed she could not be the same Minerva McGonagall we saw interacting with a younger Dumbledore in "Fantastic Beasts: The Crimes of Grindelwald" -- as if we fans wouldn't notice -- I decided that she no longer got to retcon MY head-canon. When she started contradicting earlier canon, I started choosing what I would accept.