If you’ve chosen to migrate from an existing Windows Server 2003 forest to a new forest running the Windows Server 2012 R2 operating system, one of the things that you’ll need to deal with is account migration.
Account migration is the process of moving users, groups, and computer accounts from an Active Directory domain in one forest to an Active Directory domain in another forest. Microsoft has a special tool that is freely available called the Active Directory Migration Tool (ADMT).
ADMT allows you to migrate accounts while maintaining SID history and passwords. ADMT works by creating a clone of an account. This means that during the migration process, the account is not deleted from the source forest.
Using ADMT requires a bit of preparation and obviously you’ll want to have your destination forest up and running before you start trying to move accounts across. Microsoft has published a great guide that runs to 256 pages describing how to migrate accounts from one forest to another. You can download the ADMT guide from Microsoft’s website here: http://www.microsoft.com/en-au/download/details.aspx?id=19188 and should consult it before trying to initiate a migration.
You can obtain the ADMT tool (current version 3.2) through Microsoft Connect at the following address: https://connect.microsoft.com/site1164/program8540