A thousand years is a very long time to think, in silence and solitude ... How much could Luna have suffered, while her sister reigned Equestria alone? What things could she have felt, thought and remembered?
Exile is always a long process to rethink many issues in life.
I've wondered if maybe, the death of Snowdrop was what cemented the transformation. It's just a headcanon of mine, the final moments of NMM with Snowdrop, a scene that keeps running through my head.