With SBS 2003 there is a ‘Transition Pack’ to grow out of the 75 user cap or move your servers (ie. Exchange or SQL Workgroup) to other dedicated servers. Here are links for the SBS Transition Pack. When installing the Transition Pack, make sure that the media is newer than the version you are upgrading to (ie. if the media is Transition Pack R2, then you must uninstall Windows Server 2003 SP2 before installing).
Microsoft Documentation – link
Moloy’s Blog on the Transition Pack – link
SBS page – link
