Change is bound to happen since the books are so long and full of detail that would not all fit into a several hour film. The real question is will the change be in size or will it also be in substance.
There is a story device called suspense, aka fear of the unknown or as put in Wikipedia "it consists of having some real danger looming and a ray of hope". This is a powerful experience since it plays in our mind, off our expectations of danger and is felt internally.
The last two Harry Potter books use suspense masterfully. Rowling builds a tremendous amount of suspense surrounding when and where a death eater will attack. She also builds suspense over who and how those attacks will manifest. It is that suspense and mystery that is a major driving force behind the story that drove me feverishly forward through many, many pages. It is that same suspense that is under threat in the upcoming film.
I have been quite disappointed with our modern batch of movie directors, except for M Night Shyamalan, who seem to be absolutely stupid in regards to the art of using suspense. Many films, with their producers and directors at the helm, have taken the easy way out and have replaced suspense with razzle dazzle graphics or the overly grotesque by showing details to the viewers. It appears that producer David Baron has also given into the dark side as well.
In a recent interview with the “Herald Sun,” “Harry Potter” producer David Baron revealed details of an extra scene to be inserted into “Half-Blood Prince,
[spoiler alert, although very vague]
The extra scene is rumored to be an attack by the Death Eaters. This would normally not be an issue since even those who have not read the books could guess that those attacks would increase but in this case the location of the attack and circumstances would destroy a powerful suspense element that is also used in the final book as a very powerful, shocking, scene. If they put in this scene they would be removing a very powerful story element and experience.
[spoiler alert over]
I find it hard to believe that Rowling would allow this to happen so does it mean that she has come up with an even more brilliant way to build that suspense and surprise for the last two films. Only time will tell...
(via MTV Movies Blog)