I had to use this step to be able to remove exchange 2003 from SBS 2003 before I demoted it.
Delete the Enterprise Recipient Update Service.
You will not be able to delete the Recipient Update Service (Enterprise Configuration) by using Exchange 2003 or Exchange 2000 System Manager. Perform the following steps to delete the Recipient Update Service (Enterprise Configuration) by using ADSI Edit (AdsiEdit.msc):
Open ADSI Edit, expand Configuration, expand CN=Configuration,CN=, expand CN=Services, expand CN=Microsoft Exchange, expand CN=, expand CN=Address Lists Container, and then select CN=Recipient Update Services.
In the result pane, right-click Recipient Update Service (Enterprise Configuration), click Delete, and then click Yes to confirm the deletion.
You will need the tool: