The book progresses in a linear fashion. It moves forward chronologically from beginning to end. The filmmakers, however, chose to start the narrative years after the events of the book and situate the entire film as a flashback.
This decision to mess with the timeline affects the overall tone of the film in several ways. In the book, things become steadily worse as the book progresses, with no relief in sight. This forces the reader to feel and sympathize with the plight and hopelessness of the animals. The ending of the book, which sees Napoleon and his fellow pigs effectively transformed into human beings, is the culmination of this progression.
It’s the absolute worst possible thing that could happen to the animals on the farm, and the book ends abruptly afterward, leaving the reader feeling frustrated and angry, which is the appropriate response to the events in the narrative.
The film takes a different perspective, one which radically affects the viewer’s impression following the film. It begins with a group of animals returning to the farm after the fall of Napoleon’s empire. These animals went into hiding following the betrayal of Boxer, and are only now returning to see what’s become of their beloved animal farm.
Knowing that there are animals who survive the events on the farm effectively hamstrings the tension of the film’s events. Sure, the viewer is still frustrated with Napoleon’s actions, but they aren’t as dramatic when you know all along that he’ll eventually fall from power and that a group of animals will ultimately survive.
This change in timeline and addition of extra events not found in the book injects something which Orwell purposely excluded from his novel: hope. The ending in the book is depressing, yes, but it’s also effective. Changing it just to appeal to a broader audience is a disservice to the ideals of the author.