How to set when chang permalink-structure?

I also need an answer to this question.

Previously, my permalink structure was in the form of:
/blog/%year%/%month%/%day%/%postname/

But I have now changed my structure to just:
/blog/%postname%/

Please advise of how I can use wildcards to create a rule for all pages this would apply to (all of my blog pages). I don’t want to have to create a single entry for each page!