A long time ago I enabled the WordPress permalink functionality so that the links for my blog articles were somewhat human readable. I used my own custom format, /%category%/%year%/%monthnum%/%day%/%postname%/.
Awhile ago I began regretting that format because if I changed the category of the article then the URL would change and any bookmarks to that article would be broken.
Today I started reorganizing a lot of categories and decided it was time to bite the bullter. I changed the permalink format to the standard /%year%/%monthnum%/%day%/%postname%/. What was so impressive is that the old URL’s still worked, they redirected to the new URL. WordPress rocks.