If you’ve changed post slugs, taxonomy slugs, or permalink structures, you likely created 404 pages (page-not-found URLs) along the way.
These 404 pages certainly aren’t helpful for visitors who stumble across them and can increase your bounce rate. It can be a particular problem if search engines, pingbacks, and/or internal and external links are sending traffic to the old URLs.
Here’s how to resolve the issue:
Stage 1: Identify 404 Problem Pages
If you’re registered with Google Webmaster Tools, you can view the list of 404 errors that the Googlebot has come across.
You can also use the 404 Notifier plugin, which can notify you via email or RSS feed whenever a 404 error is thrown.
Stage 2: Redirect
If the 404 errors are due to a change in permalink structure, upgrade to the latest version of WordPress. WordPress 2.3 and above will automatically redirect your old permalink structure to your new one.
If the 404s are because of changed slugs, or other, more complex content relocations, use the powerful Redirection plugin to point visitors and search engines to the right place with 301 redirects.