What the difference between the novel Animal Farm and Animal Farm Revisited?

Great question! George Orwell’s 1945 novel Animal Farm is an allegorical critique of Soviet Communism. Orwell tells a simple story of farm animals ousting their human owners and attempting to govern based on the principles of "Animalism." As the pigs become increasingly authoritarian, Orwell uses his tale to demonstrate the corrupting influence of power, the importance of mass-education, and the potency of propaganda, among other themes.
One of the primary ways Orwell explores these ideas is with the inclusion of the precepts of Animalism, as provided by Old Major in the opening chapter. Old Major establishes the “Seven Commandments” of Animalism as guiding principles for all inhabitants of the farm. These rules are:

1. Whatever goes upon two legs is an enemy.2. Whatever goes upon four legs, or has wings, is a friend.3. No animal shall wear clothes.4. No animal shall sleep in a bed.5. No animal shall drink alcohol.6. No animal shall kill any other animal.7. All animals are equal.

The parallels between Communism and Animalism are clear, particularly in the first and seventh commandments. What is interesting about Orwell’s story is how these commandments are slowly twisted and altered until, at the end of the story, the philosophy practiced by the ruling class of pigs is antithetical to the original intent of Animalism.
Animal Farm Revisited, written in 1971 but only published in 2008 by Irwin Friedman, is a relatively unknown pastiche of Animal Farm. In this “tribute” to Orwell, Friedman criticizes the values espoused by Western democratic powers to show that even the noblest of ideas can be corrupted for evil.
Irwin’s story closely follows the original in many respects. For example, the characters are identical. Napoleon, Boxer, Clover, Benjamin, and the whole host of familiar animals inhabit the story. There are wars with the neighboring Mr. Pilkington, as in Orwell’s original. Even the propagandistic slogans are similar. In Irwin’s Animal Farm Revisited, one of the slogans repeated by the dumber animals is “Two legs make a better leader,” which is similar to the sheep’s slogan of “Four legs good, two legs better!” at the end of Orwell’s story.
There are, however, numerous differences in the plot. Squealer, for example, is a war-mongering general rather than a smooth-talking propagandist. Clover is executed for desertion early in the novel instead of surviving to the end. In Friedman’s version, controversial changes to the laws of the farm happen with the guise of free political discussion and consensus rather than in secret.
This final example broaches the primary difference between Orwell and Friedman’s versions of the story: Friedman is deeply cynical of Western values and shapes his “Seven Commandments” to explicitly reflect this distrust. Here are the precepts followed by the animals in Friedman’s Animal Farm Revisited.

1. Fight only just wars.2. Democracy, equality & fraternity!3. Progress and enlightenment at all costs.4. Justice will always triumph.5. Speak nothing but the truth.6. Viva free enterprise (aka capitalism).7. Never lose faith.

Notice the mixture of Enlightenment values and Christian virtues. Much as the Seven Commandments are twisted in Orwell’s story, Friedman twists these Western values in his Animal Farm Revisited. Friedman’s version ends differently, but no less hopelessly, than Orwell’s.
This highlights the major difference between Orwell and Friedman: they are critiquing different things. George Orwell uses his story of Animal Farm to attack Soviet Communism, as practiced by Stalin. Irwin Friedman employs pastiche in Animal Farm Revisited to critique the core Christian, capitalist, and Enlightenment values and beliefs of the Western World.