They have changed the commandments both in their form and, even more so, in their content.
At the beginning, there were lots of commandments, and they all had to do with animals being equal and with animals not acting like human beings.
Now there are no more commandments about not acting like humans. They have all been broken by the pigs and have been erased.
Now, there is only one commandment. It is still about equality, but it no longer has the same message that the commandments used to have. Now, the only commandment is that "All animals are equal, but some animals are more equal than others."
In George Orwell's novella Animal Farm, the pigs change the Seven Commandments after they start carrying whips and walking like humans. Instead of having all seven "rules", they change them to a single idea: All animals are created equal, but some are more equal than others. This once again shows the hypocrisy of the pigs, since the phrase contradicts itself: how can all animals be equal, but some more than others? This furthers the ideas of discrimination found within the text. This is also the section of the novella where the others begin to notice that the pigs' faces have taken on an almost human look.
The pigs changed the seven commandments in three important ways. First of all, the seven commandments became one single commandment. This single commandment did not sum up the original seven. Rather, it "clarified" the seventh commandment.
"All animals are equal." became "All animals are equal, but some are more equal than others."
Also, unlike the original seven commandments, the single new commandment fails to distinguish friends from enemies, and even implies by omission that the pigs (who are now walking on two legs--a position previously used to identify the "enemy") are now, in fact, the "more equal" among the animals.
Finally, the rest of the prohibitions listed in the seven commandments no longer exist, once again implying that these are no longer necessary since those in power (the pigs) can, through their status of being "more equal" have the right to do as they please.