Migrating to Exchange 2007 in an existing Exchange 2003 organization

Currently, I’m working on a project doing a migration from Exchange 2003 to Exchange 2007 which has approximately 1500 users. The goal is not only to upgrade or migrate to the Exchange 2007 organization but also to shift the entire organization to a new datacenter all the while keeping in mind to minimize downtime. This post does not talk about the migration plan, but about a small trick that was deployed.

When migrating to Exchange 2007 from an exisiting organization, Exchange remains in a coexistence mode unless all mailboxes are migrated to 2007 and the 2003 servers are taken out. This means that during the migration process some users would be having their mailboxes on 2007 while some may be having their mailboxes on 2003. Users accessing their email via OWA, Outlook Anywhere, ActiveSync, Blackberry or RPC/HTTP should not be affected during this transition.

To overcome this, I setup the CAS role on a Virtual while keeping the HUB and Mailbox role on a physical machine. Isolating the CAS allows Exchange 2007 to proxy the requests to the Exchange 2003 organisation. Users who have their mailboxes on 2007 are presented with the new 2007 OWA login from https://yourdomain.com/owa while users with their mailboxes on 2003 still need to go to https://yourdomain.com/exchange. In order to ensure things worked right for RPC/HTTP, webmail, etc. I wrote a script and put it under the Default Site. Here is the script:

Now accessing the URL which the users originally used to access Outlook Web Access presented the users with a new interface of OWA from Exchange 2007. Users who had their mailboxes on Exchange 2007 were given the OWA 2007 experience while those who were having their mailboxes on Exchange 2003 – the request was automatically proxied to the Backend Server and they were presented with the experience of OWA 2003.