Whenever you make a change to a permalink, you risk losing the search engine listings you have for a post. Also, if anyone has linked to that particular page or post, the link now goes to a missing page. The solution is: redirection.

What is a Permalink?

First, what is a permalink? See the WordPress Codex:

Permalinks are the permanent URLs to your individual weblog posts, as well as categories and other lists of weblog postings. A permalink is what another weblogger will use to link to your article (or section), or how you might send a link to your story in an e-mail message. The URL to each post should be permanent, and never change — hence permalink.

This is an ugly permalink: http://example.com?p=1234

When do you need to do redirection?

1 – Let’s say you decide to change your permalink structure. (Why would I change the permalink structure?) You might want to move to pretty urls, for example, or to a structure with numbers instead of postname. Again, you don’t want to lose the incoming links from search engines or other pages.

2 – You decide to emphasize keywords in your permalinks. Let’s say you wrote a post called: Have Fun in the Sun. But you realized after publishing the post, you really want to sell sunscreen. So you changed the links from have-fun-in-the-sun to sunscreen-sun. You now need redirection to tell search engines to connect their listings to the new permalink.

3 – You change platforms (from plain HTML to WordPress; from Drupal to WordPress; from WordPress to Drupal) or you change the domain url. Your urls change, but you can salvage old incoming links with redirection.

How Does One Redirect a Permalink?

It used to be one had to use .htaccess for redirection.
If you use WordPress, you can use Redirection Plugin.
Drupal has various redirection modules.
Joomla has several redirection plugins.

Have you had experience with redirecting links?