Napoleon, like many historical dictators, makes great use of propaganda to establish his "reign." He rewrites history when he changes the commandments; more subtly, he recreates the character of Jones for the younger animals, and threatens them with Jones' return if they don't support him. How bad would this have been? Isn't Napoleon just Jones with two more legs, a little more weight, and an uglier face (unless you're a female pig :))? He has the dogs (his secret police) to control the animals, but he knows that it would be better if he could win their hearts and minds, even as he uses force to terrorize them.
This manipulation of the past, actually re-creation or simply creating of the past, is key to controling the people. In 1984, it becomes the famous formula: "Who controls the present controls the past; who controls the past controls the future." Imagine, not only controlling and oppressing the people, but having them love you for it!