is there a safe way to convert a pressflow website to drupal 6?
Pressflow and Drupal are API compatible, so you can simply replace Pressflow with the equivalent version of Drupal core. The only snag may come if someone has used private Pressflow functions in their module code on the site - but this is highly unlikely.

To upgrade, you can follow the normal upgrade path, which is to go to the highest version of Pressflow first, then upgrade to Drupal 7 or Pressflow 7.