Most of the changes are positive (at least from Amir's perspective). In America, Amir is able to escape his guilt regarding Hassan (for a while), and he does not have to compete for Baba's attention. Eventually, he earns Baba's approval by graduating from college and choosing Soraya to marry. On his deathbed, Baba shows interest in Amir's writing, something that Amir had longed for his whole life.
For Baba, the move to America is less positive. He goes from being one of the wealthiest and most powerful men in Afghanistan to being a struggling immigrant working at a gas station in America. He longs for his old life and friends, but the depth of his love for Amir is evident in his choice to move to America in the first place.
Interestingly, Hosseini chooses to portray America as a place where Amir prospers and grows closer to his father; yet his portrayal of Afghanistan--at least in regards to Baba's and Amir's relationship--is of a place which stirs up turmoil for the two characters. Even as an adult when Amir returns to Afghanistan, his old feelings about Baba return when Rahim Khan reveals Baba's lies and betrayal of Ali.