Thanks for the reply.
I would like to recap the procedure below just to verify that I've understood your instructions.
1. Upgrade phpBB to v3.02
2. Upgrade Photopost from 4.81 -> 6.11 by copying the installation files excluding the config files. Run upgrade.php
3. Run install file after upgrade (or can I change prefix when upgrading PP in step no.2)
4. Do I have to run any userfix script?