change date without changing permalink?

I’ve been using /YYYY/MM/post-title/ as the permalink structure for my posts, but now I’m running into a problem when I want to update old posts and change their dates as well. This is causing the URLs to change, breaking links to the old locations.

I could change my permalink structure to just /post-title/, but that appears to be retrospective, meaning it will break the URLs for all my old posts.

Is there any easy way to get around this? (Ideally without extra extensions, but I’m happy to install something if that’s the only way to do this.) Any advice would be appreciated.