A simple interpretation of Sarty's change from calling his father "Pap" to calling him "Father" might be that it signifies the young boy's coming of age. "Pap" is a diminutive, perhaps affectionate name that a child might call his father, while "Father" is more formal, indicating more maturity on the part of the one who uses it. Sarty begins to call his father "Father" at the exact point that his rebellion becomes irrevocable. With eyes newly open to his own sense of morality, he has chosen to abide by what he knows is right, and in doing so, has "betrayed" his degenerate father, warning De Spain of the man's planned treachery and putting into motion the series of events that lead to his death.
A second interpretation might be that, in going from calling his father "Pap" to calling him "Father", Sarty is trying to ascribe to his father the dignity more inherent in the second title. After calling him "Father" for the first time, Sarty repeats in his mind, "Father. My father...he was brave! He was! He was in the war! He was in colonel Sartoris' cav'ry!" Unaware that his father, corrupt even then, went to war only for the booty he might secure, Sarty struggles, with "grief and despair", to find a small measure of honor in the memory of the man who sired him.