With dcpromo.exe now being deprecrated on 2012 and 2012 R2, it is no longer obvious which method to use to demote a domain controller. Although you could resort to Powershell to get it done, there's also a way to demote using Server Manager, albeit not a very clear one.

The trick is to attempt to remove the Active Directory Domain Services role from the server, using Server Manager. When you do remove this, you will be prompted to demote the domain controller before the role can be removed, along with a link to actually perform this task.

Start Server Manager. In the upper right corner, click Manage, then click Remove Roles and Features.

Continue in the wizard that appears, making sure th  ...

When attempting to log on with a domain account on a computer joined to a domain that has both 2012R2 and 2003 domain controllers, you may encounter the following error:

Error message

unknown username or bad password

Additionally, an Event ID 4 on Source: Kerberos is logged. You can only log on using local accounts.

Solution

Mixed 2012R2 and 2003 AD environments require hotfix 2989971 to be installed on every 2012R2 DC. See the KB for a full explanation.

The hotfix requires Update 1 (2919355) to be installed first. The hotfix is also included in update rollup 2984006.