The transition process from Exchange 2003 to Exchange 2007 is a relatively straightforward process and involves the following high level tasks:
1. Prepare the Active Directory directory service, you can extend the Active Directory schema and create the Active Directory objects and universal security groups to support Exchange 2007 before you install the server roles. Run setup.com /PrepareAD from the command line on the domain controller that is the schema master at the forest root. When you run setup.com /PrepareAD, the task setup.com /PrepareLegacyExchangePermissions also runs to create the universal security group that is granted permissions to send e-mail to Exchange 2007 Hub Transport servers.