Can i allow moderators to change other users to an other user group?
Let us say from user to VIP..?
Secondary user groups has nothing to do with permissions?
I'm sorry but not. Only admins are allowed to change usergroups.
Please note that WP User Roles like subscriber, contributes, author, editor, etc don't have any affection in forum accesses. They are not used in forum. They are synches with forum Usergroups. And forum Usergroups are used to get different accesses to different forums.
Secondary Usergroups have the same power like the Primary Usergroup. It allows to set multiple Usergroups to the same user. For example if Usergroup A has access to Forum A and Usergroup B has access to Forum B and you want User AB to have accesses to both Forum A and B you should set User AB primary Usergroup A and Secondary Usergroup B or vice versa.