Why URLs shouldn't change

When websites get redesigned, updated or moved, the biggest mistake, in my opinion, is changing the URLs. 
 Here’s an essay from 1999 by Tim Berners-Lee explaining why URLs shouldn’t change. Cool URIs don’t change 
 Some of his other web style articles, can be found here. 
 http://www.w3.org/Provider/Style/ 
 Some of them are dated, but I particularly like the one on Bookmarkable Pages