Upgrading from Pressflow
The only point is that you should disable and un-install the Pressflow modules, the rest is the same that drupal 6.

The big problem will be with the custom modules. The best always is try to make the migration.

If you don't want to migrate you can use feeds to move from drupal 6 to drupal 7. But in this way you must build the site in drupal 7.

Oskar