In Animal Farm, why does Squealer take the sheep away for a week?

Squealer takes the sheep for a week to teach them to replace their chant of “Four legs good, two legs bad!” with "Four legs good, two legs better!” 

When the animals expel the humans and create Animal Farm, they create a list of Seven Commandments to live by.  The sheep find it difficult to remember all of them, and whittle it down to one concept: Four legs good, two legs bad!  This means that humans are bad, and animals are good.  (Wings are okay.) 

When they had once got it by heart, the sheep developed a great liking for this maxim, and often as they lay in the field they would all start bleating "Four legs good, two legs bad! Four legs good, two legs bad!" and keep it up for hours on end, never growing tired of it. (Ch. 3) 

During the Meetings, Snowball makes speeches that sway many of the animals.  Napoleon solves this problem by getting the sheep to bleat out "Four legs good, two legs bad!” disruptively.  Whenever Snowball carries the animals away, the sheep interfere. 

One day, Squealer orders the sheep to follow him to a far corner of the farm.  He tells everyone that he is teaching the sheep a new song, and needs privacy.  In fact, none of the animals know what they are doing. 

The sheep spent the whole day there browsing at the leaves under Squealer's supervision. In the evening he returned to the farmhouse himself, but, as it was warm weather, told the sheep to stay where they were. (Ch. 10) 

The sheep end up staying there for a week.  Then Squealer is seen to be walking on his hind legs.  He is followed by Napoleon, who has a whip in his trotter.  The sheep respond with a new chant: "Four legs good, two legs better!”  They go on like that for five minutes, and by the time they quiet down the pigs are back in the farmhouse. 

The other animals try to remember if there was a commandment about those with two legs being an enemy.  They go to look at the commandments, and find the Seven Commandments replaced with one. 

ALL ANIMALS ARE EQUAL

BUT SOME ANIMALS ARE MORE EQUAL THAN OTHERS (Ch. 10) 

From then on, it is official.  The pigs are in charge.  They walk on two legs like humans.  They carry whips like humans.  They live in the house.  There is basically no difference between the pigs and the people.