Great, another question that has been asked frequently is inplace upgrade of Exchange Server 2007 Standard edition to Enterprise. Yes you can do that, but it is a one way upgrade you can’t reverse that process.
To upgrade, goto “Exchange Management Console” | Server Configuration | Right click on server (you want to upgrade) | Select Enter Product Key | Follow the wizard | Once this is done Restart all Exchange 2007 Services.
For Exchange 2003 you can follow this link
