Setup will not allow you to uninstall Exchange Server 2003 until the Recipient Update Service has been reassigned to another server.
Figure B – the Recipient Update Service properties sheet allows you to change the Exchange server to which the Recipient Update Service is assigned If you look at the Figure B, you will notice that the Recipient Update Service has been assigned to a specific Exchange server.
The Exchange Server 2003 Setup program was designed from the very beginning to allow you to uninstall Exchange server should you have the need.
Even so, removing Exchange Server 2003 from and Exchange Server 2007 organization requires a little bit more work than what you might initially expect.
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.