I remember a story told by Scott Hahn in his study of Romans about a young woman who was attending his class. He was explaining the verses in Romans 6:15-23 and spoke about becoming a slave (doulos) to God once we have been set free from slavery to sin. She became very distraught, saying she would be no one's slave. She left the class, and dropped his course!
And, yet, this scripture captures the heart of metanoia: the change is not emancipation of our sarx but the taking on of the nous of Christ. Does this mean that we have both acting at once in our being? Yes, it seems so, especially in the end of Romans 7: "Who will rescue me from this body of death? Thanks be to God—through Jesus Christ our Lord! "
How do I live out this metanoia? By living as a slave of righteousness--obeying my kyrios who bought me with his blood and "putting on his mind". Most difficult is that my sarx is unconsciously a slave. So, it seems so much of my working out my salvation is staying awake and choosing to obey my kyrios. This requires changing my body, as the compulsion to slavery is embedded in my physical being. Thus, this metanoia requires the incarnation of Christ in my body.