Changing the date on a post… URL consequences

Isn’t WP supposed to remember old URLs if I change the date on a post? I just changed a post from last year to be:

http://www.triplepundit.com/2011/05/5-reasons-why-gri-is-the-new-leed/

The old URL would have been :
http://www.triplepundit.com/2010/06/5-reasons-why-gri-is-the-new-leed/

It’s dead now.

Shouldn’t it be smart enough to either NOT change the URL if the post is already published? or at least slap something in the htaccess to keep the old URL alive?

DO I have a bug, or is this supposed to happen?