As part of deploying Active Directory® directory service, you might choose to restructure your environment. Before doing so, you must determine when and how you want to restructure it. Organizations with an existing domain structure running Microsoft® Windows NT® Server 4.0 might perform an in-place upgrade of some Windows NT 4.0 domains and restructure others. After you deploy Active Directory, you might decide to further reduce the complexity of your environment by either restructuring domains between forests or restructuring domains within a single forest.
You can use the Active Directory Migration Tool version 3 (ADMT v3) to restructure your environment. ADMT can perform object migrations and security translation as necessary, so that users can maintain access to network resources during the restructure process. To download ADMT v3, see http://go.microsoft.com/fwlink/?LinkId=75627.
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ADMT v3, Migration Guide, Windows Server System, Windows NT 4.0