The song "Beasts of England" and the poem "Comrade Napoleon" are both meant to serve a particular political purpose among the animals, but they have very different, even opposite, objectives.

Early in the story the animals hear "Beasts of England" for the first time. It is sung by Old Major. The purpose is to teach the animals something that will rouse their spirits and unite them in a cause that is larger than their individual selves, which is revolution against man.

The sixth stanza expresses the need to work for a better future:

For that day we all must labour,
Though we die before it break;
Cows and horses, geese and turkeys,
All must toil for freedom’s sake.

The song uses the words free and freedom once each, and every one of the seven stanzas makes some type of reference to the future and how things will be when animals wrest control of their lives from man.

The poem "Comrade Napoleon," on the other hand, is an attempt to inspire obedience and reverence for the leader of Animal Farm. Unlike "Beasts of England," there is no mention of freedom or the future or insurrection. This song, rather than rallying the animals to action like "Beasts of England," tries to establish Napoleon as worthy of worship. Part of Orwell’s intention is undoubtedly to show how religion is often used to control the masses in a society.

This religious flavor is created by giving the song an almost biblical feel: the words thee and thou are each used once. Napoleon is also called Lord and giver and compared to the sun. Finally, there is no mistaking the intent of the line:

Thou watchest over all,
Comrade Napoleon.

As far as the ruling pigs are concerned, Napoleon is to seen as God by the working animals.

So, where "Beasts of England" urges animals to throw off their chains, "Comrade Napoleon" in essence asks animals to accept them and even love them.

In Animal Farm, why is "Beasts of England" replaced by the poem "Comrade Napoleon"?

In George Orwell’s novel Animal Farm, the animals are originally inspired by a revolutionary song titled “The Beasts of England.” Eventually, however, this poem is supplemented by a poem titled “Comrade Napoleon.” This later poem exemplifies one of the main transformations in the novel – the shift from an ideally communistic emphasis, in which all creatures are equal, to an unfortunately totalitarian emphasis, in which Napoleon is the object of idolatrous veneration. In the “Beasts of England” song, the animals are imagined as cooperating in the hard work that will lead to their eventual day of liberation:

For that day we all must labour,Though we die before it break;Cows and horses, geese and turkeys,All must toil for freedom's sake.

Their liberation, the song proclaims, will result in an era of material abundance:

Riches more than mind can picture,Wheat and barley, oats and hay,Clover, beans, and mangel-wurzelsShall be ours upon that day.

The tone of the poem celebrating Napoleon is significantly different.  The poem extolls Napoleon as the source of all good and happiness in the new state; the work of the other animals is not mentioned:

Thou [that is, Napoleon] are the giver ofAll that thy creatures love,Full belly twice a day, clean straw to roll upon;Every beast great or smallSleeps at peace in his stall,Thou watchest over all,Comrade Napoleon!

The shift in emphasis from the original song to the later poem symbolizes the shift, in the Soviet Union, from an early emphasis on the ideal of mutual cooperation among the revolutionaries to a later emphasis on the power of Joseph Stalin, the all-powerful dictator. Stalin, like many dictators, became the object of a cult of personality – a cult that was particularly at odds with the doctrines and ideals of communism.

Incidentally, the poem seems much weaker, aesthetically, than the song, as if Orwell is suggesting that dictatorships produce not only tyranny but also art that is artistically poor and uninspired.