The safest and simplest way to migrate is to start by adding a Windows 2008 Domain controller to your existing domain
First you need to Adprep your 2003 Domain by running
adprep /forestprep and
adprep /domainprep and
adprep /gpprep
Do this by placing the from 2008 DVD in the Windows 2003 DC - ADPREP is in the SOURCES folder on the DVD.
Once done you can run the setup program from the 2008 DVD and do an upgrade, or you can do a clean install on a new box and join the 2008 machine to the domain – the latter is my preferred option.
If you take the latter route you need to assign the 2008 new computer an IP address and subnet mask on the existing network. Make sure that the preferred DNS server on new machine points to the existing DNS Server on the Domain (normally the existing domain controller)
Join the new 2008 machine to the existing domain as a member server
From the command line promote the new machine to a domain controller with the DCPROMO command from the command line Select “Additional Domain Controller in an existing Domain”
Once Active Directory is installed then to make the new machine a global catalog server, go to Administrative Tools, Active Directory Sites and Services, Expand, Sites, Default first site and Servers. Right click on the new server and select properties and tick the”Global Catalog” checkbox. (Global catalog is essential for logon as it needs to be queried to establish Universal Group Membership)
If necessary install DNS on the new server. Assuming that you were using Active Directory Integrated DNS on the first Domain Controller, DNS will automatically replicate to the new domain controller along with Active Directory.
