There’s no direct migration path from phpBB2.
I suggest that you first upgrade to phpBB3 and then use the phpBB3 importer. That should work fine. I myself have a phpBB3 forum that was upgraded from phpBB2 years ago. The migration to Discourse works fine.
Make sure you keep a copy of your old phpBB2 database in case you need it to fixed some issues after the upgrade.
I noticed two problems that were caused by the upgrade from phpBB2 to phpBB3. I’m not sure if they were fixed in later versions of the upgrader or if they still exist. There are solutions for both of them:
- Private Messages can be fixed by enabling an option in the settings.
- Invalid votes can be fixed with this SQL script.