I am facing a similar issue so I want to make sure I understand your advice:
I backup my existing db which is 2.0.6
I delete my existing sql folder
I do a fresh install of the latest version of phpbb 2.0.15
I create the new sql as part of that process
I then use restore backup from the new forum to reload my old db data
I then must run the update_to_latest in the Install directory
Is that all correct?
all of that sounds fine, however, I don't think you need to delete your old sql folder, besides, unless you have root access to the database server, you cant access the actual database folder anyway. If you are talking about the folders in the phpbb directory, then they have nothing to do with the actual database.
also, if your database is over 2 mb in size, the restore function in phpbb won't work, so you have to use something like: