Just listened to @jeff outline his Dark Vtuber Future, and that actually happened with Kizuna Ai, who was the first vtuber to break into the mainstream in a big way. The company behind her completely misread their market, and tried to replace the original voice actress with several additional actors, and was met with major rejection from their fanbase.
The way I think about it, vtbuers are basically like wrestlers. They are real human beings performing a corporate owned character, but are also so intrinsically tied with the character that you can't disassociate it from the performer. Just like you can't have someone other than Mark Calaway be the Undertaker, you can't have another performer be Fubuki
Pretty much, I can't imagine them doing anything but retiring the character if the VA bails for one reason or another. I don't think anyone cares about the avatars themselves, it's all about what the performer has been bringing to the table. Even a soundalike would probably get sussed out very quickly if a swap was done in secret.
Yeah, since that Kizuna Ai situation I don't think any agency is willing to pull another fast one by switching the actor behind the character - Kizuna Ai never recovered from all that fall out.
That being said though, the actors will sometimes "reincarnate" because their character might be bound to an agency or group, but they as the actors might want to move on for one reason or another. Other times they reincarnate because they're joining a group or agency and often want to start fresh. Korone and Okyayu for example had their own characters that were semi well known before retiring them to join Hololive.
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