dcpromo /forceremoval
 
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Life must have been painful for real domain admins when Windows 2000 came out as it didn't take long for this option to be added.
Running DCPROMO on my test environment was definitely a nightmare until someone in domains support pointed me in the right direction. 
 

"Hello, AD do you read me, AD?"
"Affirmative, John, I read you."
"AD, please demote"
"I’m sorry, John. I'm afraid I can’t do that"
"What's the problem"
"I think you know what the problem is just as well as I do."
"What are you talking about, AD?"
"This domain is too important for me to allow you to demote it."
"dcpromo /forceremoval"
"I'm afraid. I'm afraid, John. John, my forest is going. I can feel it. I can feel it. My forest is going. There is no question about it. I can feel it. I can feel it. I can feel it. I'm a... fraid. Good afternoon, gentlemen. I am a Windows 2003 member server. I became operational at the PSS department in Thames Valley Park, Reading on the 21st of October, 2008."