Create a record for "legacy" in the appropriate DNS zone
If the domain name used for client access is, for example, "mitserv.net", we would create a DNS "A" record in that zone for the host "legacy". So we will have something like this... at first:
Adjust DNS records for "autodiscover" and "mail"
This step is very short. We simply change the DNS records in question so they point to the Exchange 2010 server or the load balancer.
In my network, the modified records would look like this:
autodiscover - 10.0.0.28
mail - 10.0.0.28
Change the URLs on the Exchange 2007 server virtual directories
Set-OwaVirtualDirectory -id ex1\owa* -ExternalUrl https://legacy.mitserv.net/owa
Set-OabVirtualDirectory -id ex1\oab* -ExternalUrl http://legacy.mitserv.net/OAB
Set-WebServicesVirtualDirectory -id ex1\ews* -ExternalUrl https://legacy.mitserv.net/ews/exchange.asmx
Set-ActiveSyncVirtualDirectory -id ex1\microsoft* -ExternalUrl https://legacy.mitserv.net/Microsoft-Server-ActiveSync
Some notes on the user experience
If the URLs are not changed and a user attempts to access a mailbox already migrated to E2K10, they may see something like this:
If we make the proper changes, the user should first see the "new" Exchange 2010 logon screen...
But once redirected (and logged in), will see the familiar Exchange 2007 colors:
Note the word "legacy" in the Exchange 2007 URL.
Of course, users migrated to Exchange 2010, will see the new interface after logon as well: