If you have to change the file names of pages in your site a 301 redirect is the only search engine friendly method. A 301 redirect is implemented in your .htaccess file. The code 301 is interpreted as "moved permanently".
Here is an example of the code:
redirect 301 /old/old.htm http://www.you.com/new.htm
Redirecting entire sites with 301
If you are changing domain names you can redirect the entire site using a 301 redirect.
Here is an example of the code to put in your .htaccess file.
redirect 301 / http://www.you.com/
When a search engine spider re-visits your website it will obey the rule indicated in your .htaccess file. There may be a transition period that may take a few weeks before search engines rankings have re-indexed and everything is back to normal.
Be sure and notify any sites that link to you about your domain name change.