There is no need for a script.. it is so easy to upgrade.
1. Backup all existing files and remove all except for the module folder containing pnphpbb. (I coppied this folder to the root then removed everything else)
2. Delete all tables from your database EXCEPT for the pnphpbb tables.
3. De-nuke your database. Basically all you need to do is remove the nuke_ prefix from each table (or what ever prefix your DB has). So you are left with tables with names like phpbb_posts etc etc etc.
4. Create a new database for a fresh phpbb3 installation
5. Upload a fresh copy of phpbb3 to your root folder
6. Run the install
7. At the end of the install click on the convert tab and run through that.
Once that has finished you should have a fully functional board with all posts and users in tact.
I have done 3 conversions so far.. and about to do my largest site (over 4000 members, 218705 posts
The only issue I have had is the time format gets screwed.. but thats a easy fix