old man >>> new man
Going back to Dawna's original proposed interpretation, I see Eustace's undragoning as a conversion experience, although not necessarily the putting off of the old man and putting on of the new man. According to Ephesians 4: 20-24 this process seems to describe something that those who have already come to Christ are encouraged to do on an ongoing basis:
Ephesians 4:20-24 (New King James Version)
20 But you have not so learned Christ, 21 if indeed you have heard Him and have been taught by Him, as the truth is in Jesus: 22 that you put off, concerning your former conduct, the old man which grows corrupt according to the deceitful lusts, 23 and be renewed in the spirit of your mind, 24 and that you put on the new man which was created according to God, in true righteousness and holiness.
Putting off the old ways and being transformed to confirm to the new by renewal and conforming to God's righteousness and holiness was a command given to the Ephesian believers.
True, when we initially choose Christ in faith we become new creatures (2 Corinthians 5:17), but until we enter God's presence we will still have to choose, moment by moment, to turn from the world, our old selfish natures, and Evil. This ongoing transformation reflects the reality of Christians today as believers strive to follow Jesus rather than the ways and values of the world. This transformation process, like our initial salvation, thankfully, is empowered by God working in us as we seek him.