You would need at a minimum a domain user account with local administrator rights on the DC. On the DC once you run DCPROM the local users and groups is only accessible through the command prompt.
Open a command prompt using the "Run as administrator" function and then run the following command.
net localgroup Administrators /add domain\user
Test the account after you've run the command by logging in with it. By default you have to be at least an admin on a DC to logon to it.