Groups disabled, but stale groups still exist

We were using AD/LDAP Sync for groups, but unfortunatly this feature is not supported by the non-profit license. After we activated the license it still worked (on v7), but after upgrade to v8.0 groups aren’t synced anymore and we are unable to change or even delete the original groups.

Any ideas how to solve this?

Best outcome would be to reenable group sync from LDAP.