This is a great question. The text says that all the animals unanimously awarded this honor upon Snowball and Boxer.
The animals decided unanimously to create a military decoration, ‘Animal Hero, First Class,’ which was conferred there and then on Snowball and Boxer. It consisted of a brass medal (they were really some old horse-brasses which had been found in the harness-room), to be worn on Sundays and holidays. There was also ‘Animal Hero, Second Class,’ which was conferred posthumously on the dead sheep.
All of this makes it clear that Snowball is the early hero of Animal Farm. Therefore, when Snowball is ousted by Napoleon, what is underlined is the rhetorical power and cunning of Napoleon.
Another important point is that Napoleon and the other pigs can manipulate the rest of the animals. They create reality with their words. This give new meaning to the words of Orwell in another book (1984):
Who controls the past controls the future; who controls the present controls the past.
In short, the creation of "Animal Hero, First Class" served a purpose in the beginning to bolster the pride of the animals, but in the end it meant nothing.