Максим Ученик,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
button, svg{
display: inline-block;
vertical-align: top;
}
</style>
</head>
<body>
<button id="btn">button</button>
<svg width="50%" viewBox="0 0 500 700">
<path d="M0,0 500,0 500,800 0,800" fill="red" id="fill" data-d="M0,0 500,0 Q470,350 500,700 L0,700"></path>
</svg>
<script>
btn.onclick = function(){
var d = fill.dataset.d;
fill.dataset.d = fill.getAttribute('d');
fill.setAttribute('d', d);
}
</script>
</body>
</html>