EDIT: sorry, I haven't noticed the rest of the string. anyway:
HTML:
<div id="test">
<span class="highlight_grey">example:</span> some additional text & <span class="highlight_grey">again...</span>
</div>
and JS:
var oldHtml = document.getElementById('test').innerHTML,
newHtml;
newHtml = oldHtml.replace(/<span class="highlight_grey">(.*?)<\/span>/gi, '[highlight color="default"]$1[/highlight]')
alert(newHtml);