The problems

If you have an established site with loyal users, why move it to a place they don't know about? If there are 200 sites out there with a link to your site, there could be another 1,000 people who have it bookmarked in their browser favourites.

Worse still you will be throwing away your position in the search engines. Suppose that all the major search engines currently have your site in their databases. If you simply move to another site without even a link from the old one, then you are lost. The search engines do not know about your new URL.

So you submit the new site to Google - and find that it gets ignored. Google is interested in sites that have links to them. So you do the sensible thing and make a link from your old site to the new one. Now at least Google admits that your new site exists. But you search for your company name or site title - lo and behold! Your old site appears at the top of the results. Your new site might not even be on the first page.

What is happening? Say your site title is Widgets in Wonderland. A Google search returns all the pages containing the words 'widgets' and 'wonderland'. Pages with those terms in prominent positions - in particular the page title - will be served first. Say there are two dozen pages in Google's index with 'widgets' and 'wonderland' in the title. This is where the famous Google PageRank comes into play. The page with the highest PageRank will come top of the pile. PageRank is calculated from links pointing to a page. All those links to your old site are boosting its position in the results.

Not only that. Suppose your old site attracted reviews with titles like 'Widgets in Wonderland: I laughed till I cried.' If these have high PageRank, they could be all over the first page of results. Now this might be flattering (if your site was meant to be funny), but they are pushing your new site down the results, while their links to your old site keep it at the top.

If your old site has been listed in the Open Directory for some time, then the listing will have propagated to the Google Directory and other ODP clones. That is a lot of links to the old URL. And it could take a long time to see them change. Some never will. Or let's say that some sites created a directory from the ODP years ago and show no sign of wanting to update it.

The answers

1. Don't move unless it is really necessary.
2. Make a clean break. Move all your content from the old to the new site in one mass move. Google does not like duplicate pages and may filter out your new site if both remain. Also by removing internal pages, you will be removing their links to the old main page, which were boosting its PageRank.
3. Leave behind the old main page with a redirect to your new site. Use an .htaccess file at the old domain to create 301 redirects from all your old pages to your new ones. Redirecting enables Google to transfer PageRank to the new site, as well as redirecting links and bookmarks. 
   
4. If you are listed in the Open Directory, go to the category in which you are listed and click on 'Update listing' at top right. Follow the instructions to give both the old and new URLs.
5. Contact the webmasters of other sites that link to yours, asking them to update the link. Find out who they are by using Market Leap's Link Popularity Analysis or Google's Webmaster Tools.
   
6. Leave your old main page there until your site logs tell you that referrals from it are insignificant. Then take it down. Google aims to filter out dead sites and expired domains, even if there are still links to them.