I'm pretty sure, if "only" one VA would leave, that specific character would get a big ending, like fighting and have to leave Equestria for good. However, it would become a huge problem if a VA went, who is speaking more than one main character.
Additionally, a little odd theory: If Twilight had to go, would Starlight become a replacement for her? If that's so than the leaving of "only" Twilight's VA wouldn't appear such a problem.
Anyway, I love every voice too much, I couldn't stand it, if they would just change the VA for one character. I would rather like a big ending for that specific character.