You could loose a lot of rank or link juice when a page is removed from your website. If you don't remove it properly you will end up with Google reporting to Webmaster Tools that you have "404 Not Found" errors. You could find this under Health > Crawl Errors.
What you want to do is to edit the .htaccess file in the root of your host to either 301 Permanent Redirect all of the old URL's to the new URL's. Go to the robots.txt file on the root as well and add the instructions not to crawl those particular url's or block them from being crawled.
What this does:
The 301 Permanent Redirect passes any rank or PR from the old page to the new page and deletes it from Google's index. The 302 Temporary Redirect passes any rank or PR to the new page but your telling Google that at some point you are going to redirect it back to the old one and doesn't remove or delete it from Google's index.
So you want to make sure you are blocking old URL's and redirecting them either with 301 or 302 Redirects and get that link juice pointed to live pages that are relevant to the old one.
