You cannot upgrade, you would need to transition to new hardware.
You will also find the licensing costs for windows 2008 and Exchange 2007 are exceptionally high compared to that of SBS
Sharepoint is a feature of SBS but if you don't use it it will not use much resource.
The way you would have to do it is:
> Install a second Domain Controller to your existing SBS network
> Install Exchange 2007
> Migrate Mailboxes/OAB/Public folder to your new server.
> Uninstall Exchange from SBS
> DCPROMO SBS to remove it from the domain.
You would then have to do the same again to move it back to your server.
There is little point in doing this if you already have SBS in place.