Automatically redirect to my new permalink structure (catergory/postname)

Hi,

Have been trying to find the solution to this on the forum but haven’t been able to. I’ve had a request to update the permalink structure on a site from /%postname%/ to /%category%/%postname/.

In order to keep the external links to the site’s posts intact, i would need to automatically redirect the old links to match the new structure (there are too many to do it manually). Simply activating the plugin and changing the permalinks doesn’t work, so what do I need to do in order for this to happen?

Many thanks

https://wordpress.org/plugins/redirection/