aristotle - 6:45 pm on Mar 22, 2012 (gmt 0)
What I found to work some years ago was to edit the navigation on every page of the old site so that no matter what you clicked you were taken to correct new page on the new site matching the navigation link that you had clicked.
This is an interesting idea. I don't remember seeing it before.
One quick question -- What would happen to the new site if the old site disappears from the web?