This table from Microsoft TechNet's section on Microsoft Exchange 2007 shows you the paths you can walk down to migrate your existing Microsoft Exchange 5.5 Server, Microsoft Exchange 2000 Server or Microsoft Exchange Server 2003 environments to Microsoft Exchange Server 2007 or how to enable coexistence (or as the Exchange team likes to call it: interoperability) with Microsoft Exchange 2000 Server and Microsoft Exchange Server 2003 environments.