Change DocumentType on a loaded Document

Is there a way to change the DocumentType on a loaded Document? I dig
through the API but couldn't find one. I understand that I might need
to recreate a Document in this case. nsIDOMImplementation interface
can help me to create a new Document with the newly modified
DocumentType. I can then import the nodes from the old document.
However, the problem I am facing then is how do I show this new
Document in the browser window? Is there a way to supply the
nsIWebBrowser/nsIDOMWindow with a parsed DOM and let it show it? Thank
you in advanced.