When it comes to large migrations, Microsoft learned a great deal from the problems administrators experienced when moving their organizations to Exchange 2000, and the software giant applied this newfound knowledge to Exchange 2003. The Exchange 2003 setup and deployment tools walk you through the installation process remarkably well, making it less likely that you'll run into problems.
But even with these enhancements, it's always a good idea for administrators to practice their deployments in a test environment that mimics the production environment as closely as possible. There are several reasons why performing a mock deployment makes sense.
First, a mock deployment helps you see how the product performs with your own eyes. Some behaviors aren't easily explainable, and even the best documentation can leave out simple tips that make the difference between a smooth migration and one perceived as shaky.
A mock deployment also gives you the opportunity to find any bugs or incompatibilities in your current setup. Every environment is somewhat unique, and what works for some people can be a disaster for others. The more testing you do—and the more realistic that testing is—the less likely you are to find out about problems at a critical moment.
But if can't accurately reproduce your Exchange topology, that doesn't mean you shouldn't get some practice before you deploy the product. Even a single server install in a classroom situation can offer some insight into how the product works and what you can expect during your real-world implementation.