*** Updated on 11/21/2007 by Wrobinson ***
Actually, this method is referred to as a transition — provided that E2K7 is installed into the existing E2K3 organization and mailboxes moved from E2K3 to E2K7. If a new AD forest and E2K7 migration is installed, then it is considered a migration because mailboxes are you guessed it — migrated.
There is no upgrade path from E2K3 to E2K7 per se; upgrades, most commonly refer to in-place upgrades where you can install a newer version on top of an existing version such as the case with E2K and E2K3.
Suffice it to say that you can transition or migrate from E2K3 to E2K7 but the term “directly” can cause confusion.
*** End Update ***
Microsoft calls this a migration, saving upgrade to refer to when you go from a non-Microsoft mail system. Now, are you running any servers that are running the 64-bit OS? Exchange 2007 requires this. The migrations I just completed was from a SBS 2000 to an Windows 2003 R2 64-bit enviroment (DC etc) with an additional 64-bit 2003 R2 with Exchange 2007. If you do a websearch, there are several white papers/ how-to’s that I used. Your migrations should be lots easier then going from a Small Business Server. You cannot just install Exchange 2007 on top of your 2003 if that is what you are thinking.
Also Exchange 2007 Standard has removed the 75 GB limit, so you don’t need to go to Enterprise unless there are other features you may need such as unified messaging or compliance logging of messages. Enterprise costs a lot more than 07 Standard.