Considering a Rollback | Microsoft Exchange Server 2003 Administrators Companion (Pro-Administrators Companion)

If you need to roll back changes, you can do so to some degree but it will require a great deal of effort. If you are performing an in-place upgrade, you must restore the server from tape backup. Be sure to replace all files. This is the only way to roll back an in-place upgrade.

To roll back partially upgraded organizations, you’ll need to do the following:

Move all mailboxes back to the 5.5 servers.
Remove entries on the Address Space tab in all connectors for all Exchange 2003 servers.
Rebuild the GWART.
Re-home public folders on Exchange 5.5 servers.

The point of no return on a rollback for partially upgraded organizations is not clearly defined, but it is something that you need to consider. The farther into the migration process you go, the more difficult rolling back to Exchange 5.5 will be. And there will be a point at which it will be easier to fix the current problems in Exchange 2003 than to roll back to Exchange 5.5.

Recommendations

Here are a few recommendations:

Perform the migration in a lab environment and take note of how you would roll back at each point along the way.
When you join an Exchange 2003 server with the Exchange 5.5 organization, you’ve really reached a watershed that is difficult to roll back because the configuration of both Exchange platforms has been replicated to the Exchange 5.5 directory and Active Directory. It is difficult, but not impossible, to cleanly remove all this replicated information.
It is very easy to delete and re-migrate user accounts using ADMT and the ADC. If you don’t like the results of your account migration with ADMT, just delete them in Active Directory and start over.
After accounts, mailboxes, and public folder information has been successfully migrated to Exchange 2003, rollback will be very time-consuming and probably not worth the effort.
In your plan, note the point at which you will no longer consider a rollback to Exchange 5.5 to be feasible and then work out contingency plans to fix unforeseen problems after that point.