JSFiddle - React, Tailwind, and code Playground

HTML

<div id="old-parent">
    <span>Foo</span>
    <b>Bar</b>
    Hello World
</div>
<div id="new-parent"></div>

CSS

#old-parent {
    background-color: red;
}

#new-parent {
    background-color: green;
}

JavaScript

var newParent = document.getElementById('new-parent');
var oldParent = document.getElementById('old-parent');

while (oldParent.childNodes.length > 0) {
    newParent.appendChild(oldParent.childNodes[0]);
}