What if Rowling writes a guide to her characters in which she gives new “back story” to the characters?
That too will not matter . . . anymore than I care much about the “Lost Books” (really his notes) that the Tolkien family keeps publishing from the author of Lord of the Rings insofar as it could possibly change the meaning of Tolkien’s main work. The text is fixed and it is as it is. The fact that Tolkien had other ideas about Frodo, Merry, or any other characters is important to discuss how the story came to be, but does not change the meaning of the text, if there is no explicit (or even hint) of the “new” matter.
This seems to be at extreme odds with how most Christians view the Old Testament in light of the New (and, as an aside, how Mormons view the Old and New Testaments in light of the Book of Mormon). It's pretty clear that Christians do hold that the words of the Old Testament have different meanings than Jews attribute to them.
(Via The Panda's Thumb.)