It is commonly asserted that St. Augustine underwent a major shift in his theological positions, but when did this occur?
According to St. Augustine, this shift occurred shortly after he was consecrated as a bishop in A.D. 395 or 396.
In St. Augustine's Retractiones, he states that he himself presented an insufficient view of grace in his own work Expositio quarundam propositionum ex epistula ad Romanos, written in A.D. 394.
St. Augustine also explains that he developed his full blown doctrine of grace (what we might call "Augustinianism proper" when he wrote his first book to Simplicianus. This book was written by St. Augustine in 396 or early 397.
We observe St. Augustine criticizing the work he had written in A.D. 394 and explaining that he underwent a major theological shift in A.D. 396. This doesn't mean that everything written by St. Augustine before 396 is all wood, hay, and stubble, but it provides us with a good reference point.