So based on what you suggest, I will have to reinstall the existing SSO type at the primary site as "Install basic vCenter Single Sign On", then select the "Create the primary node for a new vCenter Single Sign On installtion"?
I suppose there is no way to modify the original configuration?
Planning a Single Sign On Deployment
Before you install Single Sign On, consider carefully the future requirements for the deployment to determine whether a multisite or high-availability deployment is appropriate. If you install a Single Sign On instances in basic mode, you cannot later promote the instance to a multisite node
Bummer