There has always been a question How to Migrate SBS 2008 to Windows 2008 Server? I tried to put up some steps which will help in migrating SBS 2008 to Windows Server 2008 .
Step 1: Backup the SBS 2008 Box (Make sure SBS 2008 is in Healthy State)
Step 2: Introduce Windows 2008 to the SBS 2008 Domain
Use the below link to make it an additional domain controller in the same domain as SBS 2008 Installing an Additional Domain Controller
Step 3: Introduce another Windows 2008 as a member server
-Install Windows 2008 on another new box
-This Windows 2008 server would be used for Exchange server 2007 installation as it is not recommended that you run Exchange on a domain controller
-Use the below link to make this Windows 2008 as a member server and join it to the SBS 2008 domain
Introduce a Windows Server 2008-Based Member Server
Join a Domain from a Workgroup
Step 4: Install Exchange 2007 on the Windows 2008 member server
-Install Exchange 2007 on the Windows 2008 member server
How to Perform a Typical Installation Using Exchange Server 2007 Setup
Step 5: Remove POP3 connectors from SBS 2008 because these are not supported in Exchange 2007
Step 6: Public Folder Replication from SBS 2008 to Exchange 2007
-Start replicating the public folders between the Exchange 2007(part of SBS 2008) and the standalone product of Exchange 2007
How to Configure Public Folder Replication
-After replicating public folders, move the Public folder content
How to Move Public Folder Content from one Public Folder Database to Another Public Folder Database
Step 7: Move Offline Address Book from SBS 2008 to Exchange 2007
How to Move the Offline Address Book Generation Process to Another Server
Step 8: Move Mailboxes from SBS 2008 to Exchange 2007
How to Move a Mailbox Within a Single Forest
Step 9: Similarly, all the other server data(WSS, WSUS, SQL) can be moved to the standalone server.
Step 10: Transfer the FSMO roles
-Transfer the FSMO roles to the Windows 2008 which is running as an additional domain controller.
-From this point on, you have 21 days to complete the migration.
Using Ntdsutil.exe to transfer or seize FSMO roles to a domain controller
Step 11: Uninstall Exchange 2007
How to Completely Remove Exchange 2007 from a Server
Step 12: Demote the SBS 2008 as a domain controller
Demote a domain controller
Step 13: Format the SBS 2008 box