Migrate module can be alternate Drupal upgrade process, not just for importing foreign content
The originating purpose of the Drupal Migrate module is to import content from elsewhere. Where? Wordpress .. etc. Any place outside of Drupalsphere. The good thing is that the module is fairly open about what it will import, and therefore it's possible to import into a Drupal site from a Drupal site. And that by doing so, one can upgrade a Drupal site without going through the official Upgrade process.
Why might you want to do this?
It lets you start with a clean database, and perhaps even different View definitions, different content type names, different fields, etc.
I find that after awhile Drupal sites might build up some muck in the database, and this lets you start with a fresh database rather than have the Official Upgrade Process do its best trying to upgrade muck.
Anyway.. here's a few pages to refer to
http://drupal.org/sandbox/mikeryan/1234554 - a complete module for doing upgrades from Drupal 5, 6, 7 into a Drupal 7 site.
http://drupal.org/migrate - Handbook pages