Napoleon changes throughout the book as he gains power. In the beginning, he is just another pig, although he does have remarkable qualities. Napoleon is described as “pre-eminent among the pigs.”
Napoleon was a large, rather fierce-looking Berkshire boar, the only Berkshire on the farm, not much of a talker, but with a reputation for getting his own way. (ch 1)
Thus Orwell already sets up Napoleon as a pig to watch. Napoleon is bigger, and already a bit of a bully. Napoleon is not “vivacious” like Squealer. This foreshadows the fact that Squealer will be Napoleon’s mouthpiece.
It does not take long for Napoleon to start positioning himself for power. After the animals revolt, Napoleon begins to take on a leadership role.
Napoleon then led them back to the store-shed and served out a double ration of corn to everybody, with two biscuits for each dog. (ch 2)
Napoleon is being clever. He is already catering toward the dogs, whom he will need later as a private security force. He is also leading the animals to think that he is their provider and protector. This is a bold move.
The changes in Napoleon pave the way for the pigs to turn into human-like creatures. Napoleon, along with Snowball, is the first to push in the door of the farmhouse and enter. Throughout the story Napoleon changes by acting more and more human. He sleeps in a bed, drinks alcohol, wears a hat, and even begins to walk on two legs. "Near the end of the novel, he stands on two legs, just like the men he had previously denounced, and announces that Animal Farm's name will revert back to Manor Farm." (enotes character analysis, Napoleon)
With each step, he becomes more and more powerful until he acts just as abusive and controlling as the humans.