Some sequences may need to be changed or even replaced due to the narrative constraints. Since only the camera and the characters' language serves as the sole narrator of a movie, it can be quite difficult to introduce all of the events from a book.
The words or emotions can be easy transferred to sentence. But to a frame is difficult and expensive. So it is better to avoid transferring all the scenes back. It is better to go back. Now the technology has made so many improvement.
It costs a lot of money to match the characters as told in the book because the cast also has to follow what is there. If it is different then it must be repeated until everything is exactly the same as what it should be.
There is so much in books that they cannot adapt it exactly how it is in the book. They have to cut a lot of things due so that they don't have a movie that lasts 2 days or something. Some of the things in a lot of books do not translate well to the screen and often times they have to cut scenes that help a book's pace but slow a movie's pace. If they tried to make a book in the Harry Potter series and kept in every single thing that was in the book, we'd STILL be watching the 3rd one. If we even got to the 3rd one yet. There is a ton of stuff in all those books and the further the series went along the longer the books got, imagine trying to it everything in the Goblet of Fire book into the movie, it would need 3 movies to do it all. I mean they already had to turn the final book into 2 movies just to fit what they were able to fit as it is.