Prior solution: The records are unable to merge without correcting the issue because the records are converted.
To correct the issue.
1. Select the Relationships tab on the constituent record
2. Then select Group Member (notice the current group memberships)
3. Select the Group
4. Select the Members
5. Remove the membership and the history for the member that is not primary
6. Then merge the constituent records
Connect and collaborate with fellow Blackbaud users.