The solution can be found in one single RedirectMatch line. This line should be placed above the WordPress code, or in the Apache config. The line is the following:

RedirectMatch 301 /([0-9]+)/([0-9]+)/([0-9]+)/(.*)$ 
http://www.example.com/$4

This matches every URL which starts with a date, and permanently redirects it to the new URL for this post. Ofcourse you could be running your blog in a subdirectory, like I am, in that case, the line would be:

RedirectMatch 301 /subdir/([0-9]+)/([0-9]+)/([0-9]+)/(.*)$
http://www.example.com/subdir/$4

This way, all your incoming links will still work, and thus count for SearchEngines and you have the benefits of the new URL structure.

Changing your permalink structure is a post from Joost de Valk's Yoast - Tweaking Websites.A good WordPress blog needs good hosting, you don't want your blog to be slow, or, even worse, down, do you? Check out my thoughts on WordPress hosting!