Recently I took over hosting duties for a phpbb forum for a group of friends, and friends of friends. It’s not a large forum, but it isn’t small either. The database with search index was about half a gig.
Thankfully the conversion process can be run alongside the existing forum. Granted you should disable the old forum when you are upgrading, but to test the process out it is handy to not have to take the board down. Unfortunately for me the conversion process that comes with PHPBB 3.0.1 was consistently failing. Turning on all the php error reporting resulted in nothing as to why the script was failing. After some searching I read some people having problems with 3.0.1’s conversion had success upgrading to 3.0.0. The process wasn’t flawless, needing to be babysat for the occasional 500 error that thankfully can be overcome with a simple reload of the script. After a couple of hours it could at least complete the process.
I did try immediately upgrading to 3.0.1, but the update package didn’t like something and ended up leaving the board in an unusable state. Attempts to manually update things couldn’t save it. So the process was started over again and left at version 3.0.0 for the time being.
This information might be useful to someone else having problems upgrading a version 2 phpbb forum to a version 3. Good luck, dealing with their update process is a painful reminder of how much nicer it is to migrate a Rails app to a newer version.