This topic explains how to use Exchange Server 2007 Setup or unattended installation to install the first Microsoft Exchange Server 2007 server in an existing Exchange organization.
You can't perform an in-place upgrade from Exchange Server 2003 or Exchange 2000 Server to Exchange 2007. However, you can install an Exchange 2007 server into the existing Exchange organization and then move the Exchange resources, such as mailboxes, public folders, and connectors to Exchange 2007. After you perform this procedure, you will be running in a coexistence mode. You can maintain this mode for an indefinite period of time, or you can immediately complete the transition to Exchange 2007 by moving all resources from Exchange 2003 to Exchange 2007 and then decommissioning the Exchange 2003 servers.