RegEx Required For WordPress URL Redirection

With new installation of WordPress, I started Permalinks as "/%year%/%monthnum%/%day%/%postname%/", but now I've decided to change it to "/%postname%/"

Existing URLs
Code:
[url]
New URLs
Code:
[url]

but I don't want to break existing URL, so I decided to have a new rule defined to do this, here is how my .htaccess currently look...

Code:
RewriteEngine On
RewriteBase /

RewriteRule ^([0-9])/([0-9])/([0-9])/([0-9a-zA-Z-]+/)$ /$4 [R]

#uploaded files
RewriteRule ^(.*/)?files/$ index.php [L]
RewriteCond %{REQUEST_URI} !.*wp-content/plugins.*
RewriteRule ^(.*/)?files/(.*) wp-content/blogs.php?file=$2 [L]