I like the changes made to the series with this movie that led into the rest of the series, but why did the studio/producers decide to change/evolve the overall look and feel of the series starting with Prisoner of Azkaban? Why did they not want to continue the consistent style and tone of the first two films?
Note: I do not like the changes made but that is neither here nor there as that is not the answer to your question.
The answer to your question is this had a new director, Alfonso Cuarón, (the first two were directed by Chris Columbus) and the Alfonso decided to change things because he needed to put his *stamp* on it.
That is also why in Goblet of Fire the kids had long hair because a third director (Mike Newell) thought he should put *his* stamp on it.
I think the same person, David Yates directed the last 4, but for me it was already damaged because while the characters were the same the films still seemed like they were from several different series.
Edit: Ok, maybe i did add some about my reasoning for not liking the changes. My apologies. :)
To be frank, the tone of books 4-7 were considerably different from the initial three too. I think it ended up working out with Dumbledore's recasting too lol.
While POA has more even pacing, better performances from the main leads, and tries to work as a film in its own right rather than a highlight reel of the book moreso than the first two movies, it does suffer from jarring continuity errors. Like, did Hagrids hit really need to be moved? The Womping Willow wasn’t nearly as menacing as in COS either.
You nailed exactly why I think POA is my favorite. I like the entire series let me be clear but I agree that this one works as a film first and an adaption second. Where as I agree the first two feel like the cliff notes from the book. The atmosphere is cool and all but I value the film working by itself first.
Well the books change gradually in tone, from magical kiddie fluff of the first book, to Harry and Hermione spending half the last book homeless and desperate.
So the films had to gradually take a darker and more adult tone, with the stakes being higher and the dangers more real in every conflict, and with the kids having less and less support every year, until they had none. I think "Goblet of Fire" is where shit got real in the books, with a terrified and helpless Harry watching Voldemort resurrected before his eyes, but I'm okay with the films taking a darker tone one earlier, as the dark tone really worked for a story about Dementors and betrayal.
And the books aged with the audience. A ten-year-old who read the first Harry Potter book when it came out, would have been college age when the last book went on sale.
The books really are stellar, so clever in so many ways!