If for any reason, your group ID#s in your old threads version are not sequential, they get messed up upon import.
Example old threads:
1 = Admin
2 = Mods
3 = Users
4 = Guests
5 = Custom1
7 = Custom2
When imported they become
1 = Admin
2 = Mods
3 = Users
4 = Guests
5 = Custom1
6 = Custom2
Now the USER_GROUPS table will keep the old number system (ie nobody will have 6 and some users will have group id 7 even though there is no longer a group 7).
But the FORUM_PERMISSIONS table will reflect all the correct group numbers (ie anything that was a 7 would be properly changed to a 6)
What I did to solve it - I renumbered all the groups to use the old group ID numbering system. Which fixed the user's that the groups were assigned to. But then we had to manually redefine all permissions for forums (which was still easier than having to put all sorts of people in and out of groups).
