Upgrade Drupal 6 to 7 using Drush 5 in Windows
That functionality is now available as a separate module: Drush Site Upgrade


  The new home for site-upgrade drush command, which streamlines the major version upgrade process for Drupal.


There's also some reassurance that this will work with the latest version of Drush:


  IMPORTANT: Drush Sup 7.x-2.x requires Drush 5 or later.