The real trick to removing Exchange Server 2003 from your organization is reassigning the Recipient Update Service. Setup will not allow you to uninstall Exchange Server 2003 until the Recipient Update Service has been reassigned to another server. This is ironic since Exchange Server 2007 doesn't even use the Recipient Update Service. Even so, this task must be performed.
Reassigning the Recipient Update Service is fairly easy. Begin the process by opening the Exchange System Manager. Now, navigate through the console tree to Recipients | Recipient Update Service.
There are two listings for the Recipient Update Service. There is an enterprise listing, and a domain listing.
At this point in the process, you must right click on the enterprise listing for the Recipient Update Service, and then choose the Properties command from the resulting shortcut menu. When you do, the Exchange System Manager will display the Recipient Update Service properties sheet.
You will notice that the Recipient Update Service has been assigned to a specific Exchange server. By default, the server that is listed is an Exchange 2003 server. You must click the Browse button, and then choose an Exchange 2007 server from the list. After doing so, click OK to close the Recipient Update Service properties sheet.
Now it's time to turn our attention to the domain listing for the Recipient Update Service. We will do the exact same thing for this listing that we did for the enterprise listing. Right-click on the domain listing and choose the Properties command from the resulting shortcut menu to reveal the Recipient Update Services properties sheet. Now, click the Browse button next to the listing for the Exchange server, and choose an Exchange 2007 server from the list. Click OK to close the properties sheet, and were done.
Now that the Recipient Update Service has been reassigned you should have no trouble uninstalling Exchange server 2003. Simply run the Exchange Server 2003 Setup program, and then use the Change/Remove option to complete the uninstallation process.