So, in an ideal world you'd have a spec for all pages in your site. You would also have a test infrastructure that could hit all your pages to test them.
You're presumably not in an ideal world. Why not do this...?
Create a mapping between the well
known old URLs and the new ones.
Redirect when you see an old URL.
I'd possibly consider presenting a
"this page has moved, it's new url
is XXX, you'll be redirected
shortly".
If you have no mapping, present a
"sorry - this page has moved. Here's
a link to the home page" message and
redirect them if you like.
Log all redirects - especially the
ones with no mapping. Over time, add
mappings for pages that are
important.
