why does diana rename dunstan? how is this a turning point for his character?

Diana renames Dunstan to Dunstable because she finds the name"Dunstable" old and stale. She effectively baptizes Dunstan into a new man. The name change is really an extension of Diana's impact on Dunstan. She ushers Dunstan into a new world free of war. She initiates him into matters of sex, he no longer is a virgin, and gives him a sense of purpose he did not have before. Although Dunstan refused to marry Diana, he comes back alive and, despite his injuries, a whole man.