Change Rails Session ID

We need to change users' session id for a production db migration
(there's really not a better option).

Is there a clean way of doing this.

This works but it's such a hack and there are 2 Set-Cookie headers,
the old one and this new one, which is bad:
      headers["Set-Cookie"] = "_our_session_id=2324234234243; path=/;
expires=Wed, 01 Jun 2011 07:00:00 GMT"

Is there a better way of doing this by using the session object or
something that respects the Rails framework better?