Mac OS X Server 10.0.3: Do not install server Over Mac OS X client
You should not install Mac OS X Server 10.0.3 over Mac OS X (client). Erase the Mac OS X (client) volume before installing Mac OS X Server on it. Erasure is an option in the Installer.
This does not apply to Mac OS X Server 10.1 or later. See the documentation included with later versions for installation options.
Warning: Be sure to back up any irreplaceable data from the destination volume before erasing it.