SBS2003 ersetzen mit einem 2008 + Exchange 2010.

Füge den 2008er Server der Domain als Member hinzu.
Treffe die Vorbereitungen auf dem SBS mittels dem 2008er (32bit) adprep.
Dann stufe den 2008 zum weiteren DC in der SBS Domain hoch, kontrolliere welchen Modus die SBS Domain hat.
Hebe den AD ForestLevel auf 2003 an.
Installiere den Exchange 2010, richte diesen ein, verschiebe die Postfächer vom 2003 auf den 2010, öffentlicher Ordner dito.

Warte einen Tag ab, deinstalliere den Exchange 2003, übertrage die 5 Rollen auf den 2008,
denke an den Wins, richte den auch auf dem 2008 ein. Dann kontrollierst du die Events auf den
Servern, demote den SBS und entferne diesen aus der Domain.

Oder etwas genauer beschrieben:

1. Add the new 2008 server as a member server in the existing SBS 2003
domain.
2. Run adprep on the SBS to update the AD schema.
3. Add Domain Controller to current domain
4. Add Exchange server to the current domain.
5. Add Security Server to current domain
6. Promote 2008 DC to domain controller in current domain.
7. Install .NET 1.0
8. Install ExPRA
9. Run Exchange best practices analyzer
10. ServerManagerCmd -i powershell
11. ServerManagerCmd -i RSAT-ADDS
12. Install IIS, and compatibility add-ons for new exchange server
13. Transition mailboxes to the new server.
14. Transition FSMO roles from SBS 2003 to 2008 server including naming
master
15. Redirect Inbound Mail to the Exchange 2007 server
16. Move public folders the Exchange 2007 server
17. Migrate DNS server
18. Finalize Exchange Server Migration
19. Move a CA to a Different Computer
20. Change login scripts
21. Move group policies etc. to 2008 server.
22. Change the domain controller that Exchange 2007 connects to
23. Remove SBS 2003 server from domain.

When we tried to migrate into EBS, we had a few different issues. All of
which prevented the initial setup of EBS. The first error occurred on the
management server, the second issue on the messaging server. They were fatal
errors that prevented the initial setup from completing. I do not remember
them exactly, but only remember it would not even setup correctly. Also, I do
not know what you are talking about, so it must not have been me. I am in TX.
It is ok about the EBS thing though. I have just successfully migrated the
SBS 2003 domain to a 2008 Standard domain and have all emails, and public
folders..... Woo Hoo!!!