Drupal 6 to Pressflow migration: MyISAM to InnoDB? I will soon be migrating from Drupal 6 to Pressflow. As Pressflow encourages InnoDB for certain tables, how can I promote these recommendations when migrating an existing site?

Is this something which update.php handles?

In addition, as my Drupal site is not the latest Drupal core version, should I update to latest core first, and then sideways to the corresponding Pressflow version, or can I go straight from 6.19 to Pressflow 6.22?
You should be fine going from Drupal 6.19 to Pressflow 6.22 but make sure you backup files and database just in case.
As for converting the tables your best doing that either through mysql cli, or phpmyadmin.

I would treat these as two separate jobs: First, migrate to Pressflow and confirm the site is working; then, secondly, start moving tables.