How to change your website url / address.

You should minimise problems by moving an exact copy of the site and making no changes to it. This will help you to see if your change of address was a fuck up rather than wondering if its your new website design.

Then you need to do 301 redirects from page to page, on a linux platform, its typically done in .htaccess.

Then all of your rankings should start to reappear for your new site.  Change all  the links you can to point towards the new website.

Then go into your Webmaster Tools and go to the change of address function and let google know where your website is moving to.

Here is a complete guide to changing address.

http://www.google.com/support/webmasters/bin/answer.py?answer=83105

Changing your html file names and not loose credit.

If you are changing file names, therefore your webpage url changes, the existing links coming in will be wasted and the authority on that page will be lost. So you need to create a .htaccess file to let the search engines know, that you have permanently moved that webpage and give it the new address. Its very easy to do. Just google  "301 redirects htaccess" if your website is on a window server you will find it more difficult. Here is a quick guide.

http://www.isitebuild.com/301-redirect.htm