Although it is unintentional, the first major corruption of the Seven Commandments of Animalism comes when Snowball, with good intentions, creates a distilled law: "Four legs good, two legs bad." While it helps to instill the ideals of Animalism in all the animals, even those without much intellect, it starts the path of simplification that ends with the seven rules converted into one rule: "Four legs good, two legs better."
The first proper corruption of the Commandments comes when the pigs move into the farmhouse, sleeping in beds, directly against the original wording of the laws:
"...You did not suppose, surely, that there was ever a ruling against beds? ... The rule was against sheets, which are a human invention... You would not have us too tired to carry out our duties? Surely none of you wishes to see Jones back?"
(Orwell, Animal Farm, msxnet.org)
With this change, it becomes obvious to the pigs that they can slowly alter the rules and as long as Squealer appeals to their fear and hatred of Farmer Jones, the other animals will forget the original wording and believe the new wording. More changes follow, giving the pigs the ability to take on human attributes while keeping the other animals oppressed. This sets up the farm to change from a community of equality to a dictatorship, and all done with the implicit approval of the other animals, who cannot think far enough ahead to see what these changes will mean.