two permalink structures, how to repair

For years, I’ve always had the permalink structure of “Day and Name.” At some point, and I don’t know when, why, or how, the structure was changed to “Post name.”

I noticed this when links to posts in my blog were coming up ‘not found.’ Now I suppose that if I were to change the structure back to “Day and Name” that the other links will be ‘not found.’ WTF, is there a “fix” or plugin to get this organized again?