In T. C. Boyle’s "Greasy Lake," the lake is a central character in the story. How does the lake change from the beginning of the story to the end?

From the beginning to the end of "Greasy Lake," the lake changes from a place of peace to one of fear. Ultimately, however, it's not really the lake itself who changes but the men who go there. By the end of the story, the lake is not the refuge it once was.
Jeff, Digby, and the narrator go to Greasy Lake to escape the pressures of city life. They want to hang out, get high, and enjoy being young. Each of them has a persona that they've adopted as renegade troublemakers. When they get into a fight at the lake, they follow it up by almost raping a girl. They're threatened with death by someone who arrives as they're pinning her down.
At this point, the narrator scrambles to hide in the lake. But the lake doesn't offer much in the way of safety or peace. There's a body floating in it; the narrator can hear the car being destroyed by the other people. The men manage to escape but decide to reject the opportunity to do drugs and party with two girls who arrive.
By the end of the story, the lake is a place to escape. It's no longer a safe haven of fun and enjoyment. It has a darker tone and is probably not a place where any of the characters will feel safe again. This change is foreshadowed at the beginning of the story, when the narrator says the Native Americans named the lake Wakan, for its clear waters, but now it's become murky and polluted, with refuse and glass littering the banks.

At first, the lake is a place where the narrator and his friends enjoy all the carefree and semi-wild carousing of youth: they enjoy the lake as a natural place, a backdrop to drinking beer, smoking pot, watching girls, and, as the narrator puts it, "howl[ing] at the stars" while listening to loud rock and roll. The lake, as a character, is innocent.
By the end of the story, the lake is no longer so carefree anymore. In it, the narrator has experienced a kind of baptism that takes him from innocence to experience. Most specifically, he encounters a dead body in the lake, and suddenly, this carefree place is where he comes face to face, literally, with mortality. He also watches his mother's car get demolished and realizes that, in the grand scheme of things, he is not as wild and tough as he had believed. The lake changes in the course of the story from a place to escape reality to a place where the narrator encounters reality full-force.