RewriteXmlNamspace.cs

/*
 * Replaces the default xmlns namesapce for the given XElement and its descendants
 * with the specified target namespace
 */

private static XElement RewriteNamespace(XElement element, XNamespace ns)
{
  RewriteChildNamespace(element, ns);

  // cannot change ns of root element directly so must rebuild it
  return new XElement(ns + element.Name.LocalName,
    element.Attributes().Where(a => a.Name != "xmlns"),
    element.Elements()
    );
}

private static void RewriteChildNamespace(XElement element, XNamespace ns)
{
  foreach (var child in element.Elements())
  {
    RewriteChildNamespace(child, ns);
    var a = child.Attribute("xmlns");
    if (a == null)
    {
      // change XName of element when xmlns is implicit
      child.Name = ns + child.Name.LocalName;
    }
    else
    {
      // remove explicit xmlns attribute
      a.Remove();
    }
  }
}