Options to migrating Windows Server 2003 to 2008
Actually, to migrate from Windows Server 2003 domain to Windows Server 2008 domain, there are two options:
1.ADMT if the customer would like to create a new domain and new forest.
2.Upgrade if the customer would like to keep the current domain structure;
Option #1: Use ADMT to migrate to a new forest
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This involves moving all your resources from Windows Server 2003 domain to a new and fresh Windows Server 2008 domain. There is a tools called Active Directory Migration Tool (ADMT) which is designed for this job.
As far as I know, ADMT v3.1 is designed for Windows Server 2008 AD migration. (Windows server 2008 R2, Not support)
For your information, please refer to:
http://blogs.technet.com/ad/archive/2008/03/10/admt-and-server-2008.aspx
To download the ADMT v3.1, please visit the below link:
http://www.microsoft.com/downloads/details.aspx?familyid=AE279D01-7DCA-413C-A9D2-B42DFB746059&displaylang=en
Step by Step ADMT v3.1 Guide, you can refer to:
http://www.microsoft.com/downloads/details.aspx?familyid=6D710919-1BA5-41CA-B2F3-C11BCB4857AF&displaylang=en
Option #2: Upgrade the existing Windows 2003 AD domain to Windows Server 2008
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If you want to upgrade your original Window 2003 AD to Windows 2008 AD, you can refer to this TechNet online article:
http://technet.microsoft.com/en-us/library/ee522994(WS.10).aspx