The society that Old Major speaks about and that the animals think is in their future is very different from the reality. Even in Old Major's speech, he foreshadows Boxer's future - seemingly as it would have been under Jones - when in fact, it will be the reality anyway:
the very day that those great muscles of yours lose their power, Jones will sell you to the knacker, who will cut
your throat and boil you down for the foxhounds
Even as Old Major continues: "All men are enemies. All animals are
comrades," there are questions raised about the wild animals. There will be a vote because the dogs are anxious to catch and kill them - "Are rats comrades?" The need to even ask this question is ominous and the fact that four out of all the animals feel that they should not be included - notably the dogs - foreshadows what is to come.
Old Major has a captive audience. Not one of them questions his authority or his right to address them and all the animals are comfortable with the leadership position he has assumed as he prepares them for his eventual death. He does not want them- after he has gone - to flounder and suffer. This indicates that pigs already hold an esteemed position amongst the animals and they are recognized as "the clever ones."
Boxer himself has blind faith in Napoleon - "If Comrade Napoleon says it, it must be right" and this, in itself, indicates that he is open to exploitation by the pigs. Leaders recognize weakness and,even as he is carried off to the glue factory- at Napoleon's command, this foreshadows the beginning of the end for the others as he basically holds the farm together. Napoleon's leadership position is reinforced here.
The disappearing milk foreshadows the pigs' position as, the animals are not party to the decision to keep the milk for themselves and, only later does Squealer - the master manipulator - present the ridiculous excuse regarding them being "brain" workers:
it is only the first in a series of events to establish the inequality that is developing between the pigs and other animals.
The fact that Napoleon manages to chase Snowball, the "hero" off the farm and then convince the animals that he has their best interests at heart and wants to prevent "wrong decisions" and the very fact that he has Squealer to communicate this information foreshadows his position as supreme leader.
The pigs do not hesitate to establish themselves and having initially set up in the harness room, they move to the farmhouse - surely this behavior foreshadows their superior position. The farmhouse is the place from which all the oppression eminated under Jones.
And so, the decline begins. The changing of the Commandments is a good indicator of the pigs' dominance and each change foreshadows the cementing of their position as leaders.