I think that a case can be made that when Changez's reaction to the towers falling represents a turning point in the narrative. It is at this point where Changez makes a noticeable break with his love of America. At a moment where the nation that held so much affinity for him is at its most challenging instant, Changez smiles. It's a turning point because it marks an instant in which he does not feel sad. He is filled with happiness because the discomfort he has been made to feel internally is being matched with an external result. For Changez, this marks a turning point because it is an instant where he feels that there is an equality in pain and suffering.
This instant has to be seen as a turning point because it is after this point where his emotional departure from America accelerates. From this point, he engages in reflection about who he is and how much he no longer fits into the American vision. This moment is a turning point because it marks a point in which Changez breaks with America and turns a corner in his thinking about the world and his place in it.