Content node migration to user profile in Drupal 6 I have Drupal 6.x installed where we a custom node type for author data. We are implementing a new design that designates users as authors and now assigning various roles such as author. 

We have some 600 authors nodes which we wish to migrate to the user base so that we can assign the users to articles again.

What is the best method for accomplishing this?
I'm assuming you're wanting to create new user accounts with these profile nodes, not migrate existing users linked to these profiles. Since you're using a custom node type and not core profile or Content Profile, there are many pre-existing solutions not available. 

If these author nodes are constantly changing and you need to maintain periodic synchronization before you're ready to launch, you can set a custom class with the Migration framework. This requires some work so it might be overkill here.

More than likely, I'd say the best solution is probably the User Import module with data fed in via Views Data Export from your existing D6 site. You can create a view of your profile nodes, export it to CSV, and use that as a base to create all your new user accounts with the User Import module.