Liferay DXP provides the option to specify a friendly URL for many content types as well as for pages. It is also possible to change that friendly URL, however in doing so, all links to the old URL will be broken.
The goal is to avoid this situation by keeping a history of old friendly URLs, so that they still work when changed. When an old friendly URL is used, Liferay will provide a permanent redirect to the browser (or other consumer) pointing to the new URL. That way the browser will know how to use the new URL going forward and will show it to the user in the URL box.
So, what happens if I want to reuse an old friendly URL for a new page or content? That should be possible too. When that happens, the old URL is removed from the history of the original content/page. It will still work, but pointing to the new content/page. To make sure this behavior is understood, it should be explained to users when they are changing a friendly URL, with a text similar to what follows:
The previous friendly URL will still work until it is reused for another page/content.