Hi Hazel;
Adding to Son's response, here is a code snippet in the Javascript Code Extension that should do the trick , I tested this out in my sandbox, and it seems to work.
In the extension, if you add the following code snippet, and as long your website is running Jquery, this should do what you need it to do.
I took your requirements literally, so that all you are doing is replacing the anchor tag's href attribute with the replacement href, and that is what this code will do for you.
The code to put into the extension is below.
{code:javascript}
var product_udo=utag.data["product_brand"];
var replace_href="/pws/client/sizeguides/"+product_udo+".html";
jQuery("#productSizeGuide").attr("href",replace_href);
{code}
Hope this helps guide you in the right direction.