Microsoft publishes a good article on this process: Considerations when you upgrade to Exchange Server 2003.
If the question is more a result of process on standard and enterprise, I believe you will need to follow this path: Exchange 2000 Standard Edition to Exchange 2000 Enterprise Edition to Exchange 2003 Enterprise Edition ~or~ Exchange 2000 Standard Edition to Exchange 2003 Standard Edition to Exchange 2003 Enterprise Edition.
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