What does the drunken ordeal with the pigs show what is becoming of them?
The pigs are acquiring more and more of the humans bad habits, which they originally tried to avoid. At first, Napoleon demands they do not drink, after his experience with alcohol and the subsequent sickness. But, after he recovers, he feels the drive to consume more. His fondness for it is growing, and he now demands that they produce it, and decide to use the field that was supposed to be for retired animals to graze on.
The drunkenness is mirroring Mr. Jones' binges, in which the farm suffers, as do the animals when he does not concern himself with anything but drinking.
The pigs are changing their commandments to suit their desire to drink. At first, the commandment is that they may not drink at all. After Squealer's mishap, the animals find it has been changed to no drinking in excess. Soon, what they consider to be "excess" will need to be changed, as well.