A.
Exchange 2007 is supported only in production environments running on 64-bit (x64, no Itanium); Exchange 2003 is only support in production environments running on 32-bit platforms. There's no direct in-place server upgrade 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 via the Exchange management shell environment). You should use the Exchange 2007 functionality for mailbox migrations, you can't use the Exchange 2000/2003 mailbox migration wizard to move mailboxes between Exchange 2000 or Exchange 2003 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, either before, during, or after the other Exchange 2007 server roles.
After replacing the Exchange 2003 front-end server with Exchange 2007 Client Access server, an Exchange 2003 mailbox user still can use Microsoft Outlook Web Access (OWA) to access their mailbox through the Exchange 2007 Client Access Server's /Exchange virtual directory (for example: http: //<server FQDN>/Exchange). You can't use an Exchange 2003 or Exchange 2000 front-end server to connect to an Exchange 2007 mailbox server. To get the Exchange 2007 OWA experience, you need Exchange 2007 Client Access and Mailbox servers.