Moving to Exchange Server 2007 is a big project. Even after you've gotten your budget approved and your new hardware in place and have read all the support documents Microsoft has to offer, you still have to complete a list of tasks to prepare your organization for the upgrade. Here's a brief look at what you need to do to ready Active Directory (AD) and your legacy organization for the change:
Step 1: Determine whether you can use some of your existing hardware; if so, you might be able to save some money and employ a leapfrog upgrade approach.
Step 2: Verify that your AD's schema master is running Windows Server 2003 SP1.
Step 3: Verify that every site containing an Exchange server has at least one DC running Windows 2003 SP1.
Step 4: Make sure the functional level for any domains that will contain Exchange 2007 servers is set to Windows 2000 native or higher.
Step 5: Upgrade any Exchange Server 5.5 servers to at least Exchange 2000 Server. Be aware that not all Exchange 2003 and Exchange 2000 features are supported by Exchange 2007.
Step 6: Switch your Exchange organization into native mode.
Step 7: Modify the registry of Exchange 2003 and Exchange 2000 servers to suppress link state updates.
Step 8: Finalize AD preparation by running Setup commands to set necessary permissions, extend the AD schema, and prepare AD and the domains.
Step 9: Run the Exchange Server Best Practices Analyzer (ExBPA)'s Exchange 2007 readiness check.