DC Move

Hi All.
I have an old DC( the only one) on an old domain with a database which is needed by the client pc’s. I’m about to move all the client pc’s to a new domain which has a new DC on it already. I want to demote the old DC from the old domain and join it to the new domain as a simple member server so that the client pc’s can continue to use the database after the move. What is the best way to do this?
Thanks.