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."
"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."