You don't need to cast the same actors for the same character, suspension of disbelief allows for cast swapping when needed as long as they say "this person is Savik now" I think the story was worse off for the rewrite, I didn't care who played the part as long as they were good
The only circumstance that they would have needed to get Christie Alley specifically to play Savik was in an episode of TNG in which they had cast Kelsey Grammer, it was a non speaking role and she WAS asking for way too much money for a joke appearance, possibally because Kelsey Grammer's career was skyrocketing and hers was.... something.