Changing the permalinks structure on an existing WordPress blog can be dangerous if you don’t know what you’re doing.

Once you have an established URL structure, a change can cause you to not only lose search engine visibility, but all of your PageRank will drop to zero, and all of your inbound links will no longer work. That can’t be good!

Additionally, if all of the page names in the search engines index cease to exist, then anyone that ever clicks on one of your Serps would be taken to a 404 error. That’s bad!