As we have demonstrated in one of our Core Server screencasts - How to Install Domain Controller Role on Server Core, running DCPROMO requires that you provide the necessary information either by an unattend file or by switches. By default, Core Server does not come with a DCPROMO wizard.

There are cases, when you need to demote a Server Core domain controller. Again, you will have to use the DCPROMO command. It is still a good idea, to take a look at the DCPROMO parameters, as we have demonstrated in our previous screencast. Depending on your needs, you have to decide which of them will be suitable for your scenario.
In a production environment, it is recomended, to pipe DCPROMO results into a log file and examine it at the end.

In the following screencast, you will see how to demote a Server Core, and how to check the result of DCPROMO.