This post discusses ‘how you can upgrade your existing Exchange environment to Exchange Server 2010 from a client access perspective?’ For most of you, this’ll also mean coexisting with legacy Exchange and Exchange 2010 for a period of time. Some of you may’ve environments that’ve Internet facing AD sites and non-Internet facing AD sites. As part of upgrade process, you’ll be following a model where: • Ensure all Exchange 2003 servers are at Service Pack 2 • Deploy Exchange 2010 CAS, Hub Transport, and Mailbox in the "Internet Facing AD Site" • Have legacy Exchange servers in the "Non-Internet facing AD site" (if they exist). In other words, it would look something like this for an Exchange 2003 upgrade/co-existence.
Full Article: Exchange blog