This question has come up lots of times and the general statement is that it is not trivial.  In fact the normal question is can Exchange 2007 be run on an SBS 2003 box.  The answer to this is a resounding NO.  Exchange 2007 is 64-bit and SBS 2003 is 32-bit and never the twain shall meet.   The closest you can get is installing a Windows Server 2003 64-bit system and putting Exchange on it.  You will have to buy a Windows Server license, Exchange 2007 license and CALs for each client that uses it. (note if you want Outlook Voice Access then you need Enterprise CALs even if you are only installing Exchange 2007 Standard edition).

Once this is done you need to decide if you are going to keep your Exchange 2003 box or forward ports to the new system.  If you are keeping both then you also need to be aware of (note this comes from an e-mail from Karan in our support team and should not be considered complete, but a pointer in the right direction):

Please note that Exchange 2007 would appear in a different administrative group on 2003 snap in and they will not be able to completely manage 2007 from 2003 and vice versa. ESM and EMC are very different to each other and do not allow to perform all administrative task like LCR etc. I would like to use EMC to manage 2003 and switch back to ESM as and when required.

The recommendations are:

You must use the administrative tools that are specific to the Exchange server version that you are managing.

· You manage Exchange 2007 server roles and mailbox recipients by using the Exchange Management Console or the Exchange Management Shell in Exchange 2007.

· You manage Exchange 2000 and Exchange 2003 servers by using Exchange System Manager.

· If you are using any features from Exchange 2000 that are not supported in Exchange 2003 or Exchange 2007, you must use Exchange 2000 System Manager to manage those features.

· You can manage Exchange 2000 and Exchange 2003 recipients by using a management computer on which the administrative tools for both Active Directory and the Exchange 2003 are installed.

Mailboxes that are located on Exchange 2000 and Exchange 2003 servers are also displayed in the Exchange Management Console in Exchange 2007.

· You can manage the Exchange 2000 and Exchange 2003 mailbox properties by using the Exchange Management Console in Exchange 2007.

· To move mailbox recipients from Exchange 2000 and Exchange 2003 to Exchange 2007, you must use the Move-Mailbox cmdlet in the Exchange Management Shell

Link: Http://technet.microsoft.com/en-us/library/aa998604.aspx

Additional:

http://blogs.msdn.com/mstehle/archive/2006/10/09/FYI_3A00_-Exchange-Team-Blog_3A00_-Exchange-2007_2F00_2003-System-Management-Coexistance_2E002E002E00_.aspx

Also, I have not done this yet (I'm purely running SBS Cougar on my test system).

ttfn

David