I think that one of the most profound changes in Chapter 9 is that Boxer is permanently injured while working and is carted off to the Knacker's. It is there where he dies. This is one of the most intense changes because Boxer, along with Clover, had been one of the most staunch and loyal followers of the Revolution. His commitment never wavered with his work ethic, his trust, and his fundamental desire to "work harder" or believe that "Napoleon is always right." His death creates a void that ends up being phased out or airbrushed over time, but is one that is felt powerfully in the reader's mind.
I think that this helps to bring about another significant change in the characters. In the scene where Boxer dies, all the animals gather to say their farewells to him. Yet, after this, some unsettling feeling hits them all and Muriel starts to spell out the words on the side of the truck that is taking Boxer away. She proceeds slowly, but is interrupted by, of all animals, Benjamin. It is Benjamin who tells all of the animals where Boxer is going and what is going to happen to him. Benjamin's character up to this point had been a disassociated skeptic, cynical of everything and good for responses that only indicated futility. It is this moment where Benjamin is seen as a force of change, an agent of transformation, and someone who wants more out of life than what has been given. It comes too late, as he is unable to save the life of his best friend, but is a change, nevertheless.
Outside of Boxer's death, there are other changes in the farm. The animals are getting older, and while there was talk of retirement and full salary and benefits, nothing of the sort actually happens. In fact, the farm becomes infused with a "youth movement" of pigs, with Napoleon as their presumed father. Most of the animals have forgotten about Jones, which is a reflection of how much things have changed. Moses, the raven, has also returned to resume his sermons about "Sugarcandy Mountain." His presence is still criticized by the pigs, but is given an allowance of beer to stay and continue on with his sermons.
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What Akannan said.