So after I saw the final Harry Potter film (Harry Potter and the Deathly Hallows Part 2), I went back to the text -- that is, the book. There are a bunch of differences. For instance:
In the book, Harry is looking for the diadem of Rowena Ravenclaw, which no person living has seen, and he has the idea of speaking to someone not living -- namely, a ghost. That idea belongs to Luna in the movie.
In the book, when Harry, Ron & Hermione need to escape from Gringott's, it's Harry who has the idea of grabbing on to a dragon to get them out of there. In the movie, Hermione has this bright idea.So what are we to make of these changes?
In the book, Ron & Hermione tell Harry about their getting into the Chamber of Secrets to acquire basilisk fangs (for destroying the horcruxes in which Voldemort has secreted parts of his soul); Ron says he remembered, more or less, the words in Parceltongue -- snake language -- that Harry ad uttered when he got into the chamber some volumes back. In the movie, we actually see Ron & Hermione accomplish this, and the action comes complete with some sexually-charged banter that's part of the build-up of their becoming, beyond any doubt, an item.
All of these changes make Harry not quite so much the focus, and bring our attention to other characters in the story. On the other hand, in the movie the final fight between Harry and Voldemort seems to feature just the two of them, while in the book their duel takes place at the climax of, and in the same place as, the climactic battle in which everyone is involved. So in this respect the staging is more Harry-centric.
One answer is that there is a law of conservation of Harry: if his role declines in three smallish places, it must increase to an equal extent in one prolonged, crucial scene.
A second is that there are at least two ways to tell the story that are equally correct. And if two, why not three or more? The varying details of individual tellings still converge into a single fundamental narrative, the true essence of Harry Potter.
A third is that there is no single Harry Potter story. I said that I went back to the text, meaning the book, but who is to say that the book is the text? The movie is a text too. Of course Hollywood routinely distorts the texts of the books it turns into films, but here, as it happens, J.K. Rowling is both the author of the books and a co-producer of the movie. So she is, in some sense, a creator of both. If the true Harry Potter story is the story she created, she has created it twice, and in somewhat different forms. As poets sometimes rewrite their own poems years after their initial publication, so J.K. Rowling has revisited her own story. There are, therefore, at least two authoritative versions of the Harry Potter canon.
A fourth is that there is, after all, one and only true Harry Potter story: the book. When J.K. Rowling authorized variations from the book in the movie, she wasn't creating another authoritative version. Rather, she was just like all the rest of us as we encounter her books: she was interpreting them. J.K. Rowling had no more, and no less, license to retell the Harry Potter story than we do (except under copyright law), and her film reading of her own book is only an interpretation of the true text.
All of these arguments probably have analogies in biblical studies or constitutional law -- but I won't pursue them! The movie's fun, just as the book was -- and I hope this post doesn't spoil any of it for you.