Does an easy method exist to change my WordPress permalinks from site.com/yyyy/mm/post-title to site.com/post-title?

I'm imagining a regular expression to drop the /yyyy/mm from the URL, but am not clear how to write that in Nginx. I'd love to make a change on my WordPress config, but don't want to wreck already-published URLs.

I could also cheat, by using a redirect on the 404 page...but that feels silly.