I made a mistake of not using custom permalink on my wordpress blog and ended up having 3 folders deep on my permalinks. I already have a lot of posts with permalink structures like this. I'd like to change it to /postname but afraid that the links Ive promoted with the 3 folders permalink would have 404 error in them when users click. These posts have already been indexed by Google. Please help me redirect or avoid 404 when I change the permalink structure. Thank you!!!
You can still change them into postname so don't worry. If you're concerned with 404 if they click on it then implement a 301 redirection from your old URL structures to the new one. That way, Google will notice that the old URL has been moved permanently to a new one and will not be considered duplicate.
Create a .htaccess file and use a 301 like Grimm stated. .htaccess file would look something like: Redirect 301 /oldfolder http://www.yoursite.com/newfolder That one line will direct viewers and the search engines from your existing directory structure to the one. Go into your webmaster tools and make sure you have the most recent version of your sitemap file as well. Can take up to a month, but Google will see the URL changes and change them within their index.
If you would rather not deal with the .htaccess file, you can use the Redirection plugin. You can easily create 301 redirects with it, just by pasting in the old and the new permalinks.
You need to do permanent 301 redirection from your older permalink URL to new permalink URL. Failing to do so, your visitors will get 401 error and you will lose backlink juice and PR value which will negatively effect you in Google. The only way to solve this problem is by doing permanent 301 redirect. Since, you are using wordpress and if you are not familiar with .htaccess, then use the following plugin http://wordpress.org/extend/plugins/redirection/ or http://wordpress.org/extend/plugins/simple-301-redirects/