If the namespace is different then the new node is a not a clone of the
old node.
What you can do is create a new node in a different namespace
respectively in no namespace:

      XmlDocument doc = new XmlDocument();
      doc.LoadXml(@"<foo xmlns=""http://example.com/ns1""/>");
      XmlNode foo1 = doc.DocumentElement;
      XmlNode foo2 = doc.CreateNode(foo1.NodeType, foo1.LocalName, "");
      doc.ReplaceChild(foo2, foo1);

Note that if the old node has any child elements then you need to apply
the same approach to the child elements as well as those inherit the
namespace.