You can just set a transform to flip things and then move the shape (as it’s flipped about the origin).
<svg id="bigHalfCircle" style="display: block;" xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100" viewBox="0 0 100 100" preserveAspectRatio="none">
<path transform="scale(1, -1) translate(0, -100)" d="M 0,100 C 40,0 60,0 100,100 Z"/>
</svg>