We have managed to save the previous forum posts, all 1300 of them. I am importing into the new forum. All user accounts are active so if you had an account on the old forum you can use the new one.
several months later it is done
Why did it take so long? The old forum was JForum because I am a JSP guy (PHP problems I haz had dem...) and I didn't want to use phpbb. Unfortunatel y JForum at the time only supported MySQL as a back end - and yes you guessed I am not a MySQL guy either!
The inevitable happened: MySQL started blowing huge rocks through tiny straws and the forum would lock up. Usually when I wasn't home...
In February new server so I installed phpbb and PostgreSQL. Then looked for a way to get the dumped database data back into the forum. And looked... And looked... And tried some stuff... and some more...
I found a tool that let me import basic user data from a text file a few weeks ago. From there is was easy
This week I spent some quality hours with database dumps and phmyadmin and pgadmin3 and a few bottles of wine
So yes, it is possible to migrate from JForum / MySQL to phpbb3 / PostgreSQL but you have to be a little bit nuts to try at home.