Sometimes I wonder if the pigs are really changing, or if they are simply revealing more of their true natures as the story moves along.
In the novel, the pigs start out seeming like they are on the same "side" as everyone else; particularly when Old Major and Snowball are still alive and at the farm. However, as the story progresses, we see Napoleon and the other pigs becoming hungry for more and more power than they had at the start. They begin to take advantages and privileges for themselves that the other animals don't have, and they coerce the other animals into doing all the work while they lounge around in the farmhouse. At the beginning of the revolution, the pigs are against humans and all that they stand for; as the story progresses, they come to imitate humans and live human lifestyles.
Over the course of the George Orwell novel, Animal Farm, the pigs change from being oppressed workers in a capitalist system to framers of a socialist system in which all animals would be equal, to ultimately being the oppressors in a system much like the one from which they freed themselves at the beginning of the novel.
In a capitalist society, wealth is enjoyed by the few and the means of production are also an asset of the owner. The workers themselves do not enjoy the same profit as the owner. This is the system in place on the farm under Mr. Jones. The animals produced for Mr. Jones and were treated very poorly. They did not enjoy any of the fruits of their labor. When they were no longer of any use, Mr. Jones disposed of them. After the revolution, the animals put a socialist system in place. All animals were equal and the fruit of their labor was divided among them. After some time, the pigs began to take on the practices of Mr. Jones and the animals lost their equal footing on the farm. The seven commandments were changed to read that some animals were more equal then others. The fruits of labor were enjoyed by the pigs only, while rations and living conditions for the other animals declined rapidly.
By the end of the novel, the pigs had completely transformed from the oppressed to the oppressors on the farm.