If you're using the CMS, then the URL is formed from using the "menu title" of the page.
Once a page is published then you can update the menu title without changing the URL - since we don't want to change the URL unnecessarily.
However, if you want to force a page to use a new URL, because you don't like the current one, then the trick is to unpublish the page, then change the menu title, then publish it again.
i.e., when an unpublished page is published it'll create a new URL from the menu title, but when an already published page is update it doesn't.
Joe - Voice Admin