How to Upgrade from Exchange Server Standard Edition to the Enterprise Edition?
MS KB 170280 has info on this issue:
Exchange 2000/2003 Server Standard Edition has quite a few limitations in comparison with the Enterprise Edition. Among them is the 16GB database size limit, lack of Clustering support, lack of ability to act as a Front-End server (this limit was in fact removed in Exchange Server 2003 Standard Edition) and more (read What’s my Exchange 2000/2003 Version?).
To upgrade the Standard Edition of Microsoft Exchange Server to the Enterprise Edition of Microsoft Exchange Server, run the Setup program from the Exchange Server Enterprise Edition compact disc and click the Reinstall button.
Note: Do not forget to create a full backup set of your Information Store, including all Mailbox and Public Folder stores BEFORE you begin the upgrade process.
To install any additional components or connectors included in the Enterprise Edition of Exchange Server, run the setup program again and click Add/Remove.
Note: Once upgraded you cannot downgrade Exchange Enterprise Server edition to the standard version of Exchange Server.
Note: After checking to see that the upgrade process was indeed successful you should run your backup software and create a full backup set of your Information Store, including all Mailbox and Public Folder stores, starting a new cycle of backing up the IS. Do NOT continue the old backup cycle, start a new one.