Why are changes made to the working of each commandment during the course of the novel? Animal Farm by George Orwell

The
alterations in the commandments of the animals exemplifies how power corrupts. For, once  has
eliminated  as his only competition and gained power, he has become a dictator with vicious dogs
to enforce his rules andas his propagandist to keep the other animals from voicing
objections. 

And, since Napoleon now has absolute power, the rules are bent
to fit him.  In addition, he uses trickery and the "spin" that Squealer puts upon
truth in order to enrich himself. He is so...