Question: how to demote old domain controller?
Answer: Optional: Swap the IP address. ... 
Verify that U-Move has successfully moved all DNS settings and zones to the new DC. ... 
Verify that U-Move has successfully moved all FSMO roles to the new DC. ... 
Run Dcpromo or Server Manager to demote the old DC. ... 
Reboot the demoted DC (if not done automatically.)