Replace HTML tags with DOMdocument while keeping nested tags

Question

I have a complex HTML structure but a part of it looks like this

....
<div class="otherclass"> 
   <div class="myclass">
       <b>some HTML</b>
       ...
   </div>
   <div class="myclass">
       <b>some HTML</b>
       ...
   </div>
</div>
...

I would like to transform it into

...
<div class="otherclass">
   <span>
       <b>some HTML</b>
       ...
   </span>
   <span>
       <b>some HTML</b>
       ...
   </span>
</div>
...

What I currently have is

$xpath = new DOMXpath($doc);
$elements = $xpath->query("//*/div[@class='myclass']");
foreach( $elements as $element) {

    $new = $doc->createElement('span', $element->nodeValue);
    $parent = $element->parentNode;
    $parent->removeChild($element);
    $parent->appendChild($new);

}

But this will strip all HTML inside:

...
<div class="otherclass">
   <span>some HTML...</span>
   <span>some HTML...</span>
</div>
...

I have the HTML as string as well