Migrating SBS 2003 just got easier - Software Update to Support “Join Domain” Migration of Windows Small Business Server 2003 Data and Settings to New Hardware

Windows SBS 2000/2003 enforce a limitation that permits only one Windows SBS domain controller in the domain. This software update extends the allowable grace period for the supported scenarios from 7 days to 21 days.

I suspect "Cougar", aka SBS 2008, will have the same 21 day limitation.