On the one hand, he said "In Christ, I am a new creation, the old has gone and the new has come." This seems to suggest that transformation comes in an instant. It happens, then it's over. Old is past. New is present.
But he also said, "...continue to work out your salvation with fear and trembling..." (Philippians 2:12, NIV) Further, Paul lamented the 'war' that continued to rage within him between good and evil in Romans 7.
So, transformation, it seems, BEGINS at a moment or a decision, but it doesn't end there. Transformation is ongoing. Again, Paul's words come to mind from Philippians 3
Philippians 3:12-14 (NIV) "Not that I have already obtained [perfection], or have already been made perfect, but I press on to take hold of that for which Christ Jesus took hold of me. Brothers, I do not consider myself yet to have taken hold of it. But one thing I do: Forgetting what is behind and straining toward what is ahead, I press on toward the goal to win the prize for which God has called me heavenward in Christ Jesus."
Press on toward that goal!