The car was designed for 6 V, it was built for 6 V, it operated for 60 + years just fine with 6 V.
If the car could actually vote and have a say in this discussion, I think it would prefer to stay the way it is. Everything is fine, new 6V (Optima) solves everything.
Changing everything would be your idea, not the car. Also no logic in doing that.
Looking at this another way, what would you think about having your grand kids deciding to uproot you and change everything in your life just because they wanted to.
I don't think that would go over so good.
Go out and ask the car what it wants.
Simple.