All animals are equal, but some animals are more equal than others.
These words are the final and the most important Commandment given to the inhabitants of the Animal Farm by dying Old Major. This Commandment was left intact by Napoleon and Squealer until they gradually changed the rest, less obviously important. Who cares that the pigs are now allowed to sleep in beds if the essential rules are still here? Later, the more ominous changes appeared, like “no killing” changed to “no killing without a reason”. Of course, Napoleon immediately finds plenty of reasons to get rid of those who disagree with him.
The last change was noticed by Clover - an old and not very bright mare, who worked hard for the Farm for all her life. But even she, naive and oblivious to the politics, starts to understand that something changed and now something is very wrong. She comes to Benjamin - an embittered donkey who is intelligent enough to read - and asks him if the Seven Commandments are still the same. She is very shy to ask and considers that something is wrong with her first: Clovers apologizes that her sight starts to fail her, as does her memory, so her thoughts about the Seven Commandments turned wrong might be just her own delusions.
But when Benjamin sees the wall, where the Commandments used to be, he breaks his rule not to interfere anymore and reads aloud the only Commandment that is here now and that replaced all the rules existing before: “All animals are equal, but some animals are more equal than others”. In fact, replacing the just and idealistic Commandments with only one rule, which gives to “some” animals (e.g. pigs) the unlimited power, is the clear line of transition from Socialism to totalitarian regime. If before it there was some hope that the things can return to normal, now there is not a spark of it. The Farm became as harsh and oppressive place to live as it was before the Animal Revolution.