Why does Squealer have to teach the sheep a new song before the pigs could appear on two legs in Chapter 10 of Animal Farm?
The animals have long prided themselves on walking on four legs as part of the code of Animalism, holding fast to the motto "four legs good, two legs bad," so Squealer has to be prepared to drown out dissent when he and the other pigs begin to walk on two legs. He does this by taking the sheep aside for a week to a waste area and teaching them a new song. The sheep are stupid and will blindly repeat whatever they are taught.
When the pigs appear on two legs it causes shock: Clover starts a "terrified neighing" at the sight. The animals are surprised enough to protest, but then the sheep begin a loud bleating of "four legs good, two legs better" that goes on for five minutes. Once they are done, the moment of shock and possible protest has passed and the pigs get away with the final step in becoming just like their former human masters.
Here, Orwell shows how mindless repetition of a slogan or idea can be used to suppress dissent.
When Squealer emerges walking upright on his hind legs followed closely by Napoleon, the pigs must have known that the other animals would be not only surprised but disgruntled. One of the most important of the original Commandments had been "FOUR LEGS GOOD, TWO LEGS BAD," and the pigs were obviously flaunting their newly-achieved human behavior. So, the pigs taught the sheep a new song that simply went "Four legs good, two legs better!" Napoleon and Squealer knew that the animals would better go along with this new idea once they heard the song, but to solidify their new uprightness, the Commandments had been reduced to one: ALL ANIMALS ARE EQUAL, BUT SOME ANIMALS ARE MORE EQUAL THAN OTHERS.