Just a quick note as I go through 70-410 training from CBT Nuggets. Normally whenever you access Active Directory Sites and Services you may see list of domain controllers within your site/sites:
And each of them has NTDS Settings. But what if you see domain controller without NTDS settings here? It means that some of your domain controllers was demoted/removed from environment, but whenever you do this you have to remove domain controller from your site manually so that it doesn’t presented to you in this location/console.
And, yes, to configure existing DC as Global Catalog you just go to NTDS Settings Properties and select respective checkbox:
