One of the more helpful aspects of implementing Exchange Server 2003 is the Exchange Server 2003 Deployment Guide. Microsoft has gathered the tools you need to complete a successful Exchange Server 2003 installation and packaged them—complete with step-by-step instructions and checklists for various scenarios—in one easy-to-find place.
If you've installed Exchange Server 2003, you should already be familiar with the deployment guide. You may also realize that Microsoft updates the resource frequently, so you should check Microsoft's Web site for the latest release before you use the deployment tools.
What you may not realize, however, is that the Exchange Server 2003 Deployment Guide also covers post-installation procedures, such as site consolidation and cross-site mailbox moves. (Support for this feature begins with Service Pack 1.) This resource walks you through these procedures so you don't need to search everywhere for how-to articles or research which gotchas you may encounter in your upgrade.
You can download the Exchange Server 2003 Deployment Guide, which Microsoft last updated in May 2004, from Microsoft's Exchange Server 2003 Migration and Upgrade Web page.