Reading Wordplay's blog on character arcs made me think (sort of the point, I think).
Corin (TRtS) has a very defined arc. His story is one of redemption, going from cold military commander to a man with heart and honour, but [hopefully] without losing any of his sense of self. This theme is almost the central one - the personal journal that runs parallel to the action.
But Corin isn't TRtS's central character. That's Ella.
If Corin's character arc is about redemption, then Ella's is almost about pollution. At the beginning of the story she is a very idealistic young woman - she saves Corin because she thinks that he can become more - but she doesn't really understand what the consequences of doing that will be.
Her idealism is eroded by the hard practicalities of life and war. She takes a life, something she swore never to do, and so loses her innocence. She doesn't quite swap places with Corin, but it's definitely a case of them moving towards each other than one adapting to the other.