When it comes time for your organization to start a possible Exchange 2007 implementation, the type of implementation you undertake depends on your existing solution. Microsoft uses pretty specific terminology when it comes to implementing Exchange 2007. I'll go over each of the possibilities in this tip.
Upgrade: Upgrade is a pretty general term that describes the overall process of replacing a legacy e-mail system with Exchange 2007. Normally, Microsoft uses the term "upgrade" to describe an in-place update scenario during which a newer version of a product is installed over the top of an existing product. However, this is not possible with Exchange 2007. Exchange 2007 can be installed only on a 64-bit operating system running atop 64-bit hardware. Older versions of the product are 32-bit only, so there is some element of mutual exclusivity that takes place. That said, you will see Microsoft use the term "upgrade" very loosely when it comes to Exchange 2007.
Migration: Microsoft considers a migration to be the scenario in which you move to Exchange 2007 from a completely different system, such as Notes, or from Exchange 5.5. Exchange 2007 does not support any scenarios in which Exchange 5.5 coexistence is required.
Transition: The process of moving from Exchange 2000 or Exchange 2003 to Exchange 2007 is considered to be a transition. In this scenario, Exchange 2007 is installed on new hardware in the existing Exchange organization and user mailboxes are moved from the legacy system to the new servers.
The terminology differences might not seem that significant, but will definitely help as you make your way through the Exchange 2007 documentation.