I'm not quite clear on exactly which change you are referring to, but the opening of the story is a descriptive scene reflecting on the beauty of the Salinas River Valley in California in the 1930s. This very soon gives way to a description of George and Lennie, the two vagrants, migrant workers on their way to the next job, just pawing their way through the Great Depression like millions of others.
I believe Steinbeck draws a very clear contrast here between the beauty of the land and the desperation of the two main characters. There's lines such as this in the opening:
"...on the valley side the river is lines with trees - willows fresh and green with every spring..."
Then when he progresses to describing George and Lennie you see phrases like "restless eyes" "sharp, strong features" "shapeless of face" and "he walked heavily", all clear contrasts to the beauty Steinbeck describes in his opening paragraphs.
He wants us to appreciate both the beauty of his native California, and the desperate situation the characters are in.
John Steinbeck's use of dreams, the unconscious, reccurring myths, and symbolic characters are characteristic of what Carl Jung, the founder of analytical psychology, called the "visionary" style. Steinbeck himself once noted that realism is the surface form for his interest in psychology and philosophy. His opening scene in Of Mice and Men illustrates just this realism that suggests dreams, the unconscious, the evolutionary and the visionary.
Fed by the Salinas River, the Salinas Valley is lush and fecund, and is known as "The Salad Bowl" of the world because of its lettuce and many vegetables. Yet, located in the central Salinas Valley is the town of Soledad, which means solitude or loneliness. These contrasting settings suggest Steinbeck's call of a lost Eden to the alienated itinerant workers who enter the scene. There seems, too, an almost evolutionary stage to Lennie, who is described in animalistic terms. For instance, after dragging his feet "as a bear drags his paws," when he comes to the river, Lennie places his face in the water and "dabbled his big paw in the water." In contrast to Lennie, George stares morosely at the water, and, perhaps sensing the conflicts to come as he and Lennie will again experience a change in their lives, George complains of having to have Lennie around:
"I could get around so easy and so nice if I didn't have you on my tail."
Right before the "day was going fast now," with a little wind that dies down, George discovers a dead mouse in Lennie's pocket and throws it across the pool into some brush. But, his anger fades as he realizes that Lennie does not understand and he remembers his obligation to Lennie. Then, acquiescing to the myth, George recites the dream of owning a ranch with rabbits, like the rabbits that have taken cover from the men.
In this visionary opening scene, very craftily Steinbeck has suggested the events of his entire narrative. The lost Eden, the lost dream; Soledad not far away, the men's loneliness; the dead mouse, the dead woman; the rabbits running for cover, the flight of Lennie; "the day going fast," the end of their short employment and their friendship. Because all these events are foreshadowed, the opening scene is one of changing moods from peaceful hiatus to conflict and to dream, then to nightfall.