As per
http://support.microsoft.com/kb/822179, "You can run Exchange Server 2003 on either a member server or on a domain controller. After you install Exchange Server 2003 on a server, do not change the role of the server. For example, if you install Exchange Server 2003 on a member server, do not use the Dcpromo tool to promote the server to a domain controller. Or, if you install Exchange Server 2003 on a domain controller, do not use the Dcpromo tool to demote the server to a member server. Changing the role of a server after you install Exchange Server 2003 may result in loss of some Exchange functionality and is not supported."
So, the move mailbox scenario is the safest (and supported) way to go.