I’m currently finishing off a migration from Windows 2003 SBS to Windows 2008 SBS, everything has gone well. If I was to demote the Windows 2003 SBS box as described in the migration guide could this be kept on the network alongside the Windows 2008 SBS box. I know that there is a grace period where you can have two SBS server on the same domain for 21 days but would the 2003 box be ok to stay on the network if it was demoted? Any help would be most grateful

You can demote it but SBS is designed to be promoted during a certain time period (i cant remember how long the grace period is) whereby it will shutdown. This is by design as SBS is supposed to be a DC as well as the FSMO role holder in the Forrest. It will happily chug away on the same network assuming you dont have any services on it like DHCP that will conflict with the other DC but its not a practical scenario. Purchase another server license and build another box. Additionally SBS 2008 premium edition comes with another license for Windows 2008. It may be worth checking what version you have purchased.