Server 2003 to Server 2012 Migration links
Step-By-Step: Adding a Windows Server 2012 Domain Controller to an Existing Windows Server 2003 network PDF: 2003AD2012
Step-By-Step: Migration of DHCP from Windows Server 2003 to Windows Server 2012
PDF: DHCP
Step-By-Step: Active Directory Migration from Windows Server 2003 to Windows Server 2012 R2 PDF: Add2012DC