Ref: Transitioning an Exchange 2007 environment to Exchange 2010
MS ExchangeTeamBlog http://msexchangeteam.com/archive/2009/11/20/453272.aspx Transitioning an Exchange 2007 environment to Exchange 2010 When you are ready to begin transitioning your organization to Exchange 2010, you must transition the "Internet Facing AD Site" that is associated with your external Autodiscover record, then regional Internet facing AD Sites, and then transition your internal Active Directory sites. It is not supported to transition an internal Active Directory site before all your Internet-accessible sites have been transitioned. The steps for introducing Exchange 2010 into the environment are: Note: These steps do not discuss how to set up your CAS2010 servers in a load balancing array. Please review your load balancing solution's instructions for how to properly create and join your CAS2010 servers in a load balancing array. 1. In order to support external client coexistence with CAS2010 and legacy Exchange in your "Intern