i saw harry potter 3 after a day of hanging out in sf and consuming insame amounts of sushi.
a few facts, since i may have been confused before:
harry potter 1 - 152 min
harry potter 2 - 161 min
harry potter 3 - 142 min
apparently, those 10 minute difference make a huge difference in how the timing and pacing of a movie will feel. i thought the sorcerer's stone was pretty well paced. the chamber of secrets felt a litlte dragging to me.
i found the third movie to be paced just a little too fast. i didn't care whether they dropped things from the book or not (i agree with metamanda that i don't need to see everything again - how they enter hogwarts, the sorting hat, etc), but it just felt like the individual shots of the movie were truncated by a few seconds that would have greatly aided in providing a sense of variety in the pacing of the movie as well as absorbing and savoring the feel of what a situation might have felt like (the sheer boredom of the experience that is the divination class, the importance of snape assigning werewolves as a topic, the sense of peace harry had in talking to professor lupin). it ended up feeling like everything was rushed and you were being strung from one event to another.
this isn't to say that it wasn't greatly entertaining. i still liked it. it definitely had great moments and highlights of humor and irony. but my main complaint is that i wouldn't have minded another 10 minutes added to that movie to slow down the pace (a couple seconds longer here, a little longer there...). i think those few seconds would have added to the absorption of the story better.
it's the opposite problem of troy, where i thought some of the shots were too long and uninformative.
oh yeah.... the cast won't be changing for at least the fourth movie. for the fifth movie, they already have the same boy for lucious malfoy, so maybe they won't change the characters after all.
i personally think that it would be to the movies' benefit to not change the cast, seeing as how they already have a good chemistry with each other and have a good understanding of their characters. it seems like you could lose greatly in changing them for the sake of something as superficial as age.
after all, they're actors. there have been plenty other situations where people have had to act ages significantly disparate from their actual (90210, where most of the cast was a decade older than the characters they were playing, the oldest being a 31-yr old when the show first started out).