I've been seeing some interviews and trailers to the movie, and it seems that several changes have been made to the story in order to bring it to the big screen, as could only be expected. In particular, more attention is paid to the climactic battle. It seems the writers of the movie are beefing up the story at the expense of the message.
Now, what do I mean by that? Author John C. Wright gave a review of Philip Pullman's books in which he explained how Pullman sacrificed his story to spread his message. C.S. Lewis (whom Pullman detests) shares a similar problem: He is writing stories bearing messages, and if there comes a time when the story and the message are in conflict, the story will lose.
If you look at the general plot of Prince Caspian- the book, I mean- you see it sets up at least superficially an expectation which it then denies in favour of its message. The story begins with the children, the kings and queens of Narnia's golden age, being summoned back to Narnia in Narnia's time of crisis. Such an extraordinary act must mean the crisis is extraordinary- it demands a Sauron, or a Satan, or even the White Witch returned from her grave to trouble the land of Narnia again. It demands sacrifice and struggle, a great battle whose outcome is balanced upon a razor's edge, and desperate acts of courage. Instead, we have a Prince who has lost his rightful throne to a usurping and foolish uncle. The battle, when it comes, is anticlimactic in the extreme, and the uncle's end is presented almost as an afterthought. In fact, the battle is nearly irrelevant, because for Lewis's message, Narnia's true crisis lay elsewhere.
The message is that Narnia's true crisis is a lack of faith. The Narnians no longer believe in great lions or long-ago kings, and that is far worse than usurping non-entities. As such, because the kings have returned, and because Aslan himself has returned, the crisis is already over. Whereas in the first book and movie Aslan leads the reinforcements into the battle and thus brings the victory, in Caspian he doesn't even bother going to battle. Instead he leads the girls on a party and celebration. As the people of Narnia realize the old stories are true and Aslan exists, the crisis ends. Faith- or rather, knowledge- is restored to the people.
So, from everything I have read thus far, the movie contains a great battle that the book did not. I can't say for certain, but if they are beefing up the story, it may very well be at the expense of the message.
I have often thought the conflict between Lewis' story and his message was a weak part of his writing. On the other hand, the reason why these books are so popular among the conservative Christian crowd is because of the message. If the message is pulled, Lewis' books are just another story.
In spite of all this, I am looking forward to seeing the movie.