Beasts of england is replaced by a short song that does not praise obedience and duty but

The answer is "the Rebellion is over and they now have the society they wanted".

George Orwell's novel Animal Farm has different anthems, most strikingly the "Beasts of England" and its later substitution "Comrade Napoleon". "Comrade Napoleon" acclaims Napoleon and doesn't speak to freedom. This change is utilized to demonstrate the debasement of the standards of the animals’ rebellion.