Hallo,
um nur replace zu testen, würde ich auf den rest verzichten.
<html>
<script>
myReplace = function() {
p1 = document.getElementById("p1");
p2 = document.getElementById("p2");
document.getElementsByTagName("body")[0].replaceChild(p1,p2);
}
</script>
</head>
<body>
<p id="p0">p0</p>
<p id="p1">p1</p>
<p id="p2">p2</p>
<p id="p3">p3</p>
<button onclick="myReplace()">myReplace()</button>
</body>
</html>
Gruß
jobo