Upgrading Domain Controller to Windows Server 2008

http://technet.microsoft.com/en-us/library/cc731188(WS.10).aspx
- adprep /forestprep
- adprep /rodcprep (if you are planning to user RODC)
- adprep /domainprep /gpprep
then use dcpromo to join to existing Domain Controller and transfer the role to new DC
http://support.microsoft.com/kb/255504

one more important thing is
"Modify Default Security Policies"
http://technet.microsoft.com/en-us/library/cc731654(WS.10).aspx
Microsoft network server: Digitally sign communications (always)
Domain member: Digitally encrypt or sign secure channel data (always)