The ultimate goal here is to take a somewhat large (about 15,000 nodes) and complex (many contrib and custom modules) site from Pressflow 6 to Drupal 7, so I am assuming I should make the shift to Drupal 6, then upgrade to Drupal 7. Is that the safest path? Is there a better way? And if this is the best way, I was curious about the strategy or things to think about when making the move from Pressflow 6 to Drupal 6.
I have seen a lot of things about going from Drupal to Pressflow but not so many going in the opposite direction. I did find this question: is there a safe way to convert a pressflow website to drupal 6?
But like everything I found about moving from Drupal to Pressflow, people just say it is a drop-in solution, but what does that actually mean? I have done a diff between base Drupal 6 and Pressflow 6 and I can see the differences. But if I do a diff on base Drupal 6 and our Pressflow 6 site there are a lot more differences and it doesn't appear that just dropping in the Drupal 6 core and overwriting what is there will turn out well (though I will have to try this as an experiment to confirm).
In short, I am looking for a more in-depth answer to how to migrate from Pressflow 6 to Drupal 6, other than just saying it is a drop-in replacement. For example, what files/dirs definitely have to be replaced, which files/dirs should you definitely not replace, what if any changes are required to the DB (my impression is that there are none), etc?