The plan was for Chang to regain the trust of the Dean (by faking Changnesia and passing off as Kevin, the nice guy everybody likes) and then destroy Greendale from within.
He basically reached that goal in the last episode of Season 4 "Heroic Origins", where he gets the chance to ruin the school but changes his mind in the last second when the study group finally accepts him.
From the episode summary on Community Wiki:
Meanwhile outside the study room, Chang is talking on his cellphone with Dean Spreck from City College. Chang assures him his plan to destroy the study group is underway.
The plan he is talking about is revealed later in the episode:
Over in the administrations office, Dean Pelton was guilted by Chang into letting him mail an important document; Greendale's lease renewal for the land the college sublets from the Arapahoe Nation.
Chang's plan is to "forget" to mail the document which would lead to the lease for the land on which Greendale is built being cancelled. The land would go back to the native tribe that owns it and the school would have to close and maybe even be teared down.
The plan fails when the Abed realizes that Chang belongs to the group and invites him to join them, which makes him abandon the plan:
Abed informs him that he was always an honorary member of the group and was instrumental in their origins. [..] He then happily agrees to join the group but asks that they stop by the post office as he needs to mail something back to Dean Pelton.
That "something" obviously being the lease renewal. (I think the plot summary is wrong here, he does not actually mail it to Dean Pelton, but the intended recipient, probably a bank or the Arapahoe Nation.)