When I ran diagnositcs for crawl errors on my site using Google Webmaster Tools, I got the following information:
In Sitemaps (91)
Not Followed (9,201)
Not Found (1,786)
When I looked at the 91 crawl errors in Sitemaps, I found that many were for posts where the original url was established using the WordPress default http://www.mysite.com/2008/06/sample-post/
I changed the Permalink structure of my site sometime in late 2009 early 2010. Using Yoasts Definitive Guide to SEO, I followed his recommendation to change the structure to /%category%/%postname%/.
After I did that I installed the Redirection plugin, and made sure that under Manage -> Redirection -> Options,both URL Monitoring select boxes were set to "Modified posts". According to Yoast, after doing those things I should have been able to change permalinks to perfectly SEO'd permalinks without having to do anything else, or worry about the search engine consequences.
Other than using the Redirection plugin to do what Yoast instructed users to do in the first part of his definitive guide to SEO, I pretty left the plugin untouched. I didn't understand how to use it and didn't understand the consequences of deactivating it.
On February 15, 2011 I changed my WordPress theme. When I did that, I deactivated all of my plugins including the Redirection plugin and started my site using a new theme. This partly explains the thousands of crawl errors I found with Google Webmaster Tools.
Now I understand how to redirect from a bad url to a good url using the Redirection plugin. But I only know how to do this one-at-a-time. Considering I have thousands of urls to fix, I was wondering if I'm doing the right thing. Can anyone shed light?