See the question and my original answer on StackOverflow

Here is how you can do it (with a sample console app but the idea is the same for Silverlight):

Let's suppose you have this HTML:

<html>
<head></head>
<body>
Link 1: <a href="foo1">bar</a>
Link 2: <a href="foo2">bar2</a>
</body>
</html>

Then this code:

HtmlDocument doc = new HtmlDocument();
doc.Load(myFileHtm);

foreach (HtmlNode node in doc.DocumentNode.SelectNodes("//a"))
{
    // replace the HREF element in the DOM at the exact same place
    // by a deep cloned one, with a different name
    HtmlNode newNode = node.ParentNode.ReplaceChild(node.CloneNode("Hyperlink", true), node);

    // modify some attributes
    newNode.SetAttributeValue("NavigateUri", newNode.GetAttributeValue("href", null));
    newNode.Attributes.Remove("href");
}
doc.Save(Console.Out);

will output this:

<html>
<head></head>
<body>
Link 1: <hyperlink navigateuri="foo1">bar</hyperlink>
Link 2: <hyperlink navigateuri="foo2">bar2</hyperlink>
</body>
</html>