When performing a forest migration from a Server 2003 to a Server 2012 R2 forest, you could manually remove each computer from the source forest and then manually join it to the destination forest.
Of course this would be pretty cumbersome if you needed to migrate several thousand computer accounts from one forest to another!
Luckily the Active Directory Migration Tool (ADMT) includes functionality that allows you to migrate computer accounts from one domain to another. There are several ways you can do this. You can choose to use the Computer Account Migration Wizard, call ADMT from the command line, or configure a script to automatically perform the migration.
Account migration includes two general steps. The first is to create a computer account object in the destination forest that is a replica of the computer account object in the first forest. The second step involves restarting the computer itself, so that it shifts from being a member of a domain in the first forest to becoming a member of the new domain in the second forest.
Needless to say, you’ll want to ensure that user accounts have already been migrated across before you migrate computer accounts across. No point joining a computer to a new forest if the person who uses the computer doesn’t have an account in that forest!
You’ll find information about migrating computer accounts on pages 148 through 153 of the Active Directory Migration Tool (ADMT) Guide, which you can download from Microsoft’s website at the following address: http://www.microsoft.com/en-au/download/details.aspx?id=19188