Relevant incoming links are the lifeblood of SEO. Thus it is very important when you do maintenance tasks on your website – such as changing the Permalink structure in your WordPress blog – to make sure old URLs still work.
There are a lot of techniques that help users access old URLs, but only one of them is pretty effective for preserving whatever link gain you’ve accumulated. And that is the use of 301 redirects. This technique can be applied to any kind of website, not just blogs.
To make sure your PR isn’t lost when you restructure old links, stick a 301 redirect from the old URL to the new one. And the easiest way to do that is by using rewrite rules in your .htaccess.
TIP: Since 301 redirects pass the accumulated link gain, think what will happen if you use them for 2 or more old URLs and set the redirect target from all of them to the same destination URL. This technique can be used as a quick and effective way to boost a new page Rank to the sum of the link popularity of all old pages that now redirect to it.
As with any SEO technique, over-doing it can hurt your website. But used responsibly it can provide the extra advantage in front of your competitors.