It does not work this way.
You need to follow these steps:
- Upgrade to AD DS 2008:
http://www.microsoft.com/en-us/download/details.aspx?id=6170
- Add a new DC running Windows Server 2008 and make it a DNS and GC server
- Transfer FSMO roles to the new DC:
http://support.microsoft.com/kb/255690/
- Demote the old DC after doing the needed checks for the DCs health and the AD replication status using
dcdiag and repadmin commands
Of course, it is recommended to backup the system state of your current DC before proceeding.
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