[Quarterbore]

Perhaps you need to describe what you have better?

The easiest fix may be write a SQL query to build a list of secondary gropus and save that to a new temporary field. Then, create a second temporary field for the primary group.

Once you QC the new fields to make sure you are comfortable with those, turn your forums off and adjust your permissions as/if necessary. Then, change the names of the usergroup fields and verify it worked. Finally, delete the old fields with the original usergroups.

Well, that is how I would do it at least but how good are you at coding?