What happened to the pigs' faces in chapter 10 of Animal Farm?
The answer to this can be found in the last few paragraphs of the whole book. Basically, the pigs' faces are changing so that they look like human beings. This is the author's way of saying that the pigs had changed so much (in terms of their behavior) that you can no longer tell them from human beings.
No question, now, what had happened to the faces of the pigs. The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
Just after the Revolution, the animals' basic philosophy was that they needed to be unlike people. All their rules were meant to make them act like animals and not people. But they've broken all the rules and are now just like people.