Changing web site, but keeping results

I am currently writing a new web site for a company, they all ready have one and google can return many result for their contents.

However the old site is directory based so their product links were in http://<server>/products/<productname>.html

and other too like downloads was in /downloads/ etc..

No I have done the new site using php and everything runs off the main index.php, if the current files were removed would this result in the search results being deleted from google as the pages are no longer there or with the links remain.

I will be putting in a .htaccess so that all failed pages are diverted to the new location. I'm just unsure if google will remove the previous results?

This is a big issue as they do NOT want to lose there result, as some are the top matchs for some of there products.

Google will drop those pages, even with the redirect. If you value those pages, you need to keep them up. Unfortunately I'm speaking from experience...

Well, what you could do is leave the page up with a message that page has moved, then search google with "link:www.url.to/the/page.html" and find out what links are heading to the page and ask for them to be updated.