Changing Group to Many Members at Once

I have searched as many resources as possible, but no luck, so here goes.

I am running a 9.54 version WebWiz forum with over 1000 members. This is a private forum and is running well. We now have a need to create various groups beyond the usual setup, so that we can further restrict individual forms, depending on group affiliation.

Since the members are already in place and have have access, what is the quickest way to assign each member the particular group he should be in.  I do have a database with user names and group assignments, but am not sure how to import those assignments to the forum.  The import tool seems to be oriented to new members, not current members.

I also want to make sure I do this correctly and not change everyone's password.

Thank you for the help.

So there is no back end job that recalculates what group people should be moved to after making system changes?  Why not have the resync option help us with this?