The different medium demands some changes, for sure. But there are changes, and then there are changes. Peter Jackson's Lord Of The Rings trilogy illustrates it well. The Fellowship Of The Ring, the first movie, has some noticeable changes from the book: some characters and episodes are omitted, things are rearranged to suit the flow of the film. But the main characters and action pieces are consistent with the book, and the overall spirit and flavor of the book are retained, and the result is a masterpiece. I can't imagine how anyone could ever do that movie any better. But then the second movie, The Two Towers, is a crashing contrast. Many changes are made: the nature of some of the characters is changed, action episodes that occur in the book are omitted while new ones are inserted that further subvert the characterizations. The whole thing emerges as a train wreck that completely misses the mark in terms of "adaptation".