Why does Squealer have to teach the sheep a new song before this event could take place? (Chapter 10 of Animal Farm)

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.