Small Business Server 2003 and Small Business Server 2008 have to be Primary domain controller in the domain Small Business Server product are an integrated product with Exchange SharePoint, and IIS installed on the same server along with Active directory
So whatever reason it might be we should not Dcpromo down Small Business Server as this will break the integration of the various installed application on Small Business Server
Hence Dcpromo down and re-promoting the same Small Business Server is not supported
If Dcpromo down has been run on the Small Business Server the only supported mean to promote it back as domain controller is to rebuild the server starting fresh
If you have and additional domain controller in the domain which is always recommended we could follow http://support.microsoft.com/?id=884453  to join Small Business Server 2003 to an existing active directory domain.
For Small Business Server 2008 http://technet.microsoft.com/en-us/library/cc546034.aspx  should be followed