If the URL keeps changing then as far as the search engines are concerned they are new pages and the old ones no longer exist. Once a page has been assigned a URL it should not change if the content changes unless the content changes so much that the page URL no longer reflects what it is about. In that case it is a new page and thus should have anew URL.
In Wordpress once you create a slug for a page it doesn't change even if you change the title of the post (I assume other CMSs work the same way). That's because once a page has been assigned a URL changing it only serves to confuse users and search engines alike. You wouldn't want people to change their names every time they change their clothes, would you?