There are a couple of very important considerations, that you should have in mind, before you proceed with your migration scenario.
–Check, and raise, if necessary, the Domain and Forest functional levels. You cannot upgrade directly from Windows 2000 mixed, or Windows Server 2003 interim domain functional levels.
–The first Windows Server 2008 Domain Controller in the forest must be a Global Catalog Server, and it cannot be a Read Only Domain Controller, RODC.
–Check the FSMO roles assignments. When you prepare the existing AD, you should run adprep /forestprep on the Schema operations master, and adprep /domainprep /gpprep on the infrastructure master.In your case as there is a single Dc you need to run on the same server.
Steps to Install Windows 2008 R2 DC
1.First prepare the domain.
Insert Win 2008 R2 DVD on windows 2003 DC and execute adprep as below
Ran D:2008DVDSupportAdprepadprep32.exe /forestprep on the server holding the Schema Master role.
Ran D:2008DVDSupportAdprepadprep32.exe /domainprep /gpprep on the server holding the domain master role.
2.Install DNS role in win2k8
Reference KB article:http://technet.microsoft.com/en-us/library/cc725925.aspx
3.Once DNS role is installed.Ran dcpromo on win2k8 R2.
Reference KB article:http://technet.microsoft.com/en-us/library/cc753720(WS.10).aspx
4.After the Win2k8 Dc promotion is completed restart the win2k8 DC.
5.You must transfer the FSMO roles to the 2008 machine then the process is as outlined at http://www.petri.co.il/transferring_fsmo_roles.htm
6.Ran dcdiag /q and repadmin /replsum on DC to check for any errors.
7.Change all of the clients (and the new 2008 DC itself), to point to the 2008 DC for their preferred DNS server this may be in DHCP options or the TCP/IP settings.