Have two permalink structures?

I have my current permalink structure, which acts like ‘http://www.patrickweber.com/blogpost’. I would like to change it to something like ‘http://www.patrickweber.com/2005/01/01/posttitle’, just so it is a little more organized.

The only problem is that I would like to retain my current permalink structure for all previous posts alongside the new permalink structure so people dont start recieving 404’s all over the place.

Another thing, will my new permalink structure make the blog slower to access due to the longer url’s in the .htaccess?