I managed now to import all old forum posts in to the new forum system.
There was an issue with the new forum trying to update all unread counts every time I moved something over.
This took forever on my system and the forum page would not come up for up to 20 minutes.
I am not sure how to fix it so that it updates the unread counts for a user without taking so much time.
However I am also not sure if anyone else has that problem.
If you are having issues, please let me know and I will try to find a solution.