Changing URL Pattern

I created an article URL pattern of just {articleAlias} and that worked fine. But now I realise that I will be adding content with other categories so I changed the pattern to {categoryAlias}/{articleAlias}.

But none of my existing articles have changed their URLs, they are still using the original pattern, new articles are successfully using the new pattern.

How do I reset all the articles to the new pattern? I've tried clearing the cache and looking for somewhere the URL might be stored but can't figure it out.

Thanks for your help.

Thanks, it must be the menu items then, they happen to have the same name.