Exchange 2007 is supported only in production environments running on 64-bit (x64, no Itanium); Exchange 2003 is supported only in production environments running on 32-bit platforms. No direct in-place server upgrade is possible. Instead, a swing approach will be taken in which a new 64-bit Exchange 2007 server will be brought into the existing Exchange organization and mailboxes and services migrated to the new server. Mailboxes can be migrated using the Move Mailbox Wizard or the Move-Mailbox command (available through the Exchange management shell environment). You should use the Exchange 2007 functionality for mailbox migrations; you can’t use the Exchange 2003/2000 mailbox migration wizard to move mailboxes between Exchange 2003 or Exchange 2000 and Exchange 2007 servers.
Depending on the number of mailboxes, if you already have a 64-bit-capable Exchange server, you could migrate mailboxes to a temporary server, rebuild the existing server with a 64-bit OS and Exchange 2007, then migrate the mailboxes back. However, this approach might be difficult with a large amount of data.
All Exchange 2007 server roles can coexist with Exchange 2003 servers. To migrate, you should install server roles in this order: Client Access, Hub Transport, and Mailbox and Unified Messaging. Edge Transport can be installed separately from the migration planning, before, during, or after the other Exchange 2007 server roles. After replacing the Exchange 2003 front-end server with Exchange 2007 Client Access Server, Exchange 2003 mailbox users still can use Microsoft Outlook Web Access (OWA) to access their mailboxes through the Exchange 2007 Client Access Server’s /Exchange virtual directory (e.g., http: //