Through the changing of the Seven Commandments, Orwell makes two points about communication in Soviet society.
Firstly, in this regime, communication becomes a tool for oppression. Notice how the Commandments are changed without any prior discussion with the other animals and without their consent. As such, Orwell highlights the relationship between language and power. The Commandments, once a symbol of freedom on the farm, are now used by the pigs as a means of social control. They give the pigs total power on the farm and, more importantly, leave the other animals helpless to change the power dynamic. Thus, language has enabled the pigs to cement their domination and absolute power.
Secondly, the changing of the Commandments makes an important point about the role of truth in communication. When the animals realize that the Commandments have changed, they blame themselves for having poor memories instead of blaming the pigs for changing the rules. Thus, the truth has become whatever the pigs claim it to be. The truth is now a propaganda tool, designed to reinforce the pig's domination on the farm.