[Mashiro]

You could obtain all previous versions of phpBB and do a version to version upgrade maybe? That would reduce the huge amount of different code and changes going from 3.0 flat to 3.3. Just an idea. I know it involves a lot of work.

Try to see whether I can dump the database contents - empty it - and import the old database. Not sure how flawed this would be.

I can't recommend this unless you are a pro in database technology. Old data might? no will not play nice with a new database. There are references, primary keys and foreign keys and etc. Dumping old database content into a new one needs a lot of planning and can still go wrong.