How should I set up my migration from Exchange 2003 to Exchange 2007?

All environments are complex and differ in their requirements.

Generally, we recommend keeping OWA 2003 and 2007 environments separate during the migration by keeping the Exchange 2003 front-end/back-end servers intact. In this scenario, an Exchange 2007 CAS and Exchange 2007 Mailbox Servers support the 2007 users, in addition to the existing Exchange 2003 Front-end and Back-end Servers.

The redirection can be configured on the CAS where requests for /exchange are redirected to the Exchange 2003 front-end servers.

To further simplify the environment, we recommend putting an ISA server out front to act both as a firewall and a proxy server. Requests received by the ISA server are sent directly to the Exchange 2007 CAS or the Exchange 2003 Front-end Server depending on the where the user's mailbox resides.
Diagrammatically, here is what the migration model would look like, with ISA 2006:

Migration Configuration for Exchange 2007

Click image to enlarge

The ISA scenario is preferred because:

It simplifies authentication and configuration
It simplifies the management of the 2003 and 2007 environments

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