We recently re-vamped a site for a client.. and as mentioned.. all the PR's dropped off.

If I can make a suggestion.. keep the key pages that have any sort of rank.. (about.html, contact.html, etc) if they exist.. and any main section pages/top level intros.. (hardware.html, products.html, etc) even if they do not exist in the new design.

The search engines will look for them on the next crawl, as will visitors who find them in the search engines.. And instead of returning 404, which tell the page does not exist.. and gets the page removed from the SE index, salvage a redirect.

without getting too techie.. IN THE PAGE.. you can use the following if the pages are php parsed;
<?php
header("http/1.1 301 Moved Permanently");
header("Location: http://www.yourdomain.com/newpage.html");
exit();
?>
This will tell the search engine the new page location.. and invisibly redirect the site visitor to the new page.

Hope this helps.. Drop me a note if you want all the gory details..