That's funny! I've actually always been okay with movies changing books, because it's a completely different medium, different pacing, etc.* However I had a similar epiphany with the HBP movie. When I was noticing the little things that were changed, e.g., Snape goes up to Harry and shushes him before killing Dumbledore, I thought of it like the way people AU things for a fanfic. "Okay, for Harry's year seven, I'm going to have x, y, z happen, so the end of year six, I'm going to have Snape see Harry in the Astronomy Tower, instead of Harry being invisible." So, yes, to me the movies are a fanfic, and no more or less privileged than any other fanfic. :D (In fact, in some ways, I don't give JKR any more or less privilege versus others who have written different versions of what happened in her setup.)

*And in the HP movie versions, I think they've been doing an excellent job of visual representation of the story, except for the streaking black smoke wizards of TOOTP and HBP, and the streaking white smoke wizards of TOOTP. What are they, suddenly Djinn or something? Watching the classes, or duelling, or the DA practices, you get that the wizards are all very human, who have to learn and practice and apply their intelligence and conscience. I think the smoke streaks diminish the message of personal responsibility, as the wizards look too superhuman to be judged by mortal terms. As well as too far beyond Harry's skillset for him to be able to fight.