Changing the commandments is one way for Orwell to communicate an important theme of the novel concerning the power of language. Whoever controls words, suggests the novel, has the power over people's reality. As propagandist, Squealer wields this power. When the pigs take all the mild and apples for themselves, Squealer explains this as an act of sacrifice: "You don't think they really wantthe milk and apples for themselves do you? They need them to strengthen their brains so they can take care of you. You don't want Jones' to come back, do you?" (the quotation is approximate). In this way, the pigs change the perception of the animals little by little throughout the novel, so that by the end, they can believe just about everything they are told, although they can't help but look back and forth between the pigs and the men in the last pages of the novel--for there they have difficulty believing what they see (words now have nothing to do with it).
The First Commandment reads, "Whatever goes on two legs is the enemy." At the end of the novel, the pigs begin walking on two legs. Therefore the commandment must be changed in their favour.
In the course of the novel, the commandments are amended to and changed without the consent of the populace. Generally, an animal will go to the barn wall to check what the commandment says and find it altered. This shows that someone (the pigs) are in power and that they have the power to change what was once considered by the whole of the Animal Farm to be unalterable and agreed upon by all.
At the end of the novel, only one commandment is there and it says, "All animals are equal, but some are more equal than others." At the end of the day, all animals are equal but the pigs are 'more' equal. This is the best example of 'double speak' available. The less intelligent animals will not think to question this idea. But in effect, in equality is inequality and the animals are in the same state as they were with Mr. Jones. The pigs are the new aristocracy.