I've heard it said before that the role of Valeris in ST6:TUC was originally intended to be Saavik, but that idea got nixed for a couple of reasons, in favor of creating a new character. But Saavik in that "traitor" role is a very compelling dramatic idea that I personally wish hadn't been scrapped.
Reason #1 that's commonly stated is that Gene Roddenberry didn't want a well-loved character like Saavik to turn out to be a traitor. I'm not sure I buy that one completely -- by that point Roddenberry wasn't much more than a glorified consultant who had very little day-to-day influence on the production of the movies. (I've heard that he pretty much hated the script anyway; one more change like that wouldn't have done that much more damage as far as he was concerned.)
Reason #2: When Kim Cattrall was cast in the role, she accepted the part only on the condition that she *not* be yet another recast of Saavik.
But my question is, why did Kim Cattrall have to be cast in the first place? I have to admit I didn't much like her in that movie. Why not go back to one of the two women who had played Saavik in the past?
I understand that Kirstie Alley was well-established on Cheers by that time, so she may have been too expensive for the movie's budget. But why not Robin Curtis? Was she unavailable? It's not like she'd retired from acting by then; she even came back to play another character on TNG at about the same time.