I set up my first WordPress, ignorantly using the default permalink structure. Later on, I set up a new WordPress site with the numeric structure. It worked well and create pretty links. So I went back to the old site and changed the Permalink options to numeric.
Alas -- after that, the server couldn't find anything that I had links to. Fortunately, when I changed back to the default permalink structure, the site resumed normal function.
Is there any trick to change the permalink structure, once a site is up and running?