After migrating my blog to Drupal, the old Wordpress style URL's were broken, as explained here. At that time, I was trying to fix them with mod_rewrite, but didn't succeed.
Fortunately I have been fiddling around with Drupal a lot lately, and I found a rather easy way to fix my old WP-style URLs: Pathauto. It was just a matter of configuring the default pattern for blog entries:
blog/[yyyy]/[mm]/[dd]/[title-raw], deleting all existing URL aliases for nodes, and bulk generate them with the new settings. Sweet.
So I resubmitted my sitemap to Google, and requested removal for the "default Drupal-style" URLs.