In this short write up I will go over steps to demote a Server 2012 domain controller.
If you have worked in Active Directory and Windows Domain Administration over the years you may recall that in previous version of Windows Server that you would use the command line tool of ‘DCPROMO’ to promote or demote a server. Since Server 2012, the use of DCPROMO has been deprecated. In fact, if you attempt to use it you will be inform of this via the Active Directory Domain Service Installed.
In Server 2012 and later versions the use of Server Manager or PowerShell is required to promote / demote a server to/from a Domain Controller (DC). Below I provide steps on how to demote a server with some illustration along the way. Also, here is a quick YouTube video on the process: https://youtu.be/sBK2_APaDdg
Log into the domain controller you intend on demoting and Launch the Server Manager, select the Manage drop down menu, select Remove roles and features.
On the server selection page, select the desired server from the pool.
On the Remove Roles and Features Wizard, un-tick the Active Directory Domain Services box
The Remove Roles and Features dialog box will open. Click Remove features
On the Remove Roles and Features Wizard dialog box Validation Results box will appear. The domain controller must be demoted before continuing. Click on Demote this domain controller.
On the Active Directory Domain Services Configuration Wizard enter the required credentials to demote this server, click Next.
You will have several removal options. From the forced remove of failed domain member, to removing of the last domain in your forest. Make the selections which is appropriate for your remove task and click Next
Finally you will arrive on the New Administrator Password, enter and confirm the new local administrator account password, click Next.
On the Review Options verify the information is correct and click Demote.
After the server has restarted it will no longer be a domain controller
And that is it.