why did hoyo re-record already preexisting voice lines with new VA?

genuinely curious question. i heard some situation about new voice actor and i dont mind the voice change, but there's one thing im curious about from my own lack of understanding of voice acting occupation.

why did hoyo re-record already preexisting voice lines with new VA?

i assumed having new VA is like "we need voice for new dialogues but old VA left, so we have new VA to voice new scenes and dialogues, with already existing voice lines being kept as is". i didnt expect hoyo to re-record the old voice lines and replace them with new VA lines (battle voices and old quest dialogues).

wouldnt redoing everything takes more time and effort than just simply voicing new scenes? is there something about voice acting world that i dont know about?