I’ve just removed a domain controller (DC) from my root domain, the very first server not only in the domain, but the forest. The roles were migrating to a newer server, far more up to the job, but it isn’t a job to be taken lightly. If you mess up the root domain, you’ve potentially got problems all the way down your domain hierarchy.
Let me explain; the primary domain controller in a domain (normally the first domain controller) hosts all the FSMO roles.