There are more differences in between the movie and the book. In general, you can observe that things that the reader has to guess in the book, being because the author wants to maintain suspense or yet because he wants us to use our imagination to think about what was happened. However in the film every success is shown to us very clearly and nothing is ‘hidden’. An example of this is when Napoleon steals the puppies for them to be his private guard. In the book the puppies were taken off the dog, but nothing was said about them being trained. However in the movie you could perfectly see what was going on: Napoleon took the puppies shortly after they were born and appeared in numerous occasions as his personal guard, accompanying him in his speeches and defending him when somebody, even Jessie, said something ‘wrong’.
Within these differences we also have the example of how the pigs trade with humans. As in the example above, in the book you have to imagine what happens (if pigs talk with the trader, do they send a middleman, how do they talk in between them?) but you can see Napoleon speak with Mr _____ about buying trading eggs and other products for whisky and we could also see that Napoleon sold Boxer just to get money to buy whisky.
Another difference, though subtle, is the language. In the book, first published in 1945, uses a difficult and complex language; while in the film language is very adapted for maybe a younger audience or so that anybody could comprehend the movie without getting bored from not knowing what the words mean.
In my opinion, I think that the changes to the plot from the book to the film make the story more understandable and maybe becoming more suitable for younger children thanks to showing what happens more clearly, perhaps not in much detail as explained in the book. In a conclusion, the plot does change somehow, whether because of the characters, events or invented events. However the movie works best because the story is explained better and you can get an idea of what the story is about very close to reading the book.