What do the reduction and alteration of commandments symbolize in Animal Farm?
Once the rules are changed to explain or justify Napoleon's actions it is symbolic of the absolute power of the leader. Like the dictatorship of Stalin, Napoleon changes the rules, which become a tool of propaganda, to be used by the state, or governing body, to suit the actions of the government.
Napoleon becomes more and more powerful, just like Stalin did after his reign of terror in Soviet Russia when he had millions killed, exiled or tortured in the name of Communism. The message that goes along with the changed rules is that Napoleon is a great leader who should not be questioned. Therefore, everything that he does, he does for the animal's benefit. The same way that Stalin did for the starving people of Soviet Russia in the years following WWI and before WW II.
The end result is the same, the leaders of the Communist state enjoy a life of luxury and privilege, the pigs, while the peasants, or the animals, barely get by.
These changes represent many key political points on Orwell's part. One is a comment on the Soviet regime. These changes symbolize how that nation's leaders rewrote the past, insisting on official versions. Another is more subtle; this shows how laws, manifestos, visions, or commandments of any sort change when they intersect with reality. In some ways they are made more realistic. After all, not all of the animals really understood the original commandments. In most ways, though, the changes show how those in power bend the laws to their needs.