g1smd - 7:38 am 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.
Either hard code the domain name in every link, or add the base tag to top of every page.
If the old site uses "includes" of any sort, you likely have a quicker job than you previously imagined.
Nowadays, adding the rel="canonical" attribute is also a good idea.