With a basic redesign you probably wont have this problem but if you have rebuilt a website even with the same content if the URL’s have changed this will have a huge effect on your search engine presence as you will be starting from scratch. Your links from internal pages may not work, links from external sites wont work and you will lose your pagerank (if it ever mattered anyway.)
A good way to keep your sites listing after a redevelopment is to use a redirect. If you are using an Apache server you can use the .htaccess file to redirect.
Redirect 301 /oldpage.html /newpage.html
But some times this isnt the best way if you have to redirect hundreds of pages. The alternative is to use a header redirect in PHP. Most web based languages such as ASP or RUBY will do the same thing but with different code.
// make this code the first line
// 301 Moved Permanently to newpage.php
Posted by Adi on January 24, 2009