During reading a article on GSR; I was stumbled when come to know that GSR can also work without SFC(Switch Fabric Card). Actually what happens when the SFC removed from the GSR Clock Schedular Card(CSC) comes into the picture. At that time all the line engines will stop work except line engine 0. So your CSC will work as SFC.
what i think that at that time all line engines are stopped and there is no need for the clock thats why clock card(CSC) falls back to SFC card mode. It means GSR can be upgraded without shutting it down.