The original company that first hired the talent will still own the copyright to any character designed for their company - they commissioned and paid for the character to be designed for commercial purposes, and that purchase is going to include the copyright to that model.
Now, if vtubers want to start commissioning models with the same name as their previous character, with extremely similar designs, played by the same person, expressing the same opinions with the same mannerisms, you start to veer into territory where companies might want to start arguing copyright in court, which is a very expensive and time-consuming legal battle that you're going to lose even if you win. I don't know if that's a risk I'd want to take if I were a talent, especially when most talents are pretty happy to just go to a new model.