Domain Controller Downgrade

Here is the situation. I have one PC running server 2000 which is configured as a domain controller with the active directory configured. There is a second server running server 2003 which is also configured to a domain controller on the same domain, and the active directory is also configured. All user profile settings and files are stored on the second server, but the active directory is stored on the second. The problem lies in that when you shut down either PC no one can log in because the active directory cannot be accessed . How do I go about down grading the windows 2000 server without having problems with the active directory. Thank You