They shouldn't have to change moves at all. We are talking simple faction change, and I'd probably say no earlier then level 50 (Once the Class Story is complete).
Think of KOTOR, You could be a Lightside Jedi and Still Use Darkside moves. That comes from training, the Side affiliation stems from the behavior of the User, and the way they fuel their moves.
Their are obviously Darkside Moves that are more Iconic, such as Force Choke, and Force Lightning, But we are talking about changing Faction, not class. So a Sith Sorcerer would Still be a Sorcerer, but would fight against then Empire and other Sith. So rather then Converting them to a Sage, Create a new Combination of "Jedi Sorcerer"
It wouldn't be to difficult i think to nut out a plausible story arc to support the change, while keeping the Classes the same. And the potential to expand on it in the future is great, If the level cap is raised and new moves are introduced, a Combination Sorcerer/Sage (Sith turned Jedi) could potentially learn later Sage moves and keep the Original Sorcerer moves, creating a whole new "Advanced, Advanced Class"