Maganti, your assumption is incorrect. If you have a reservation from A to C with boarding at B, then you can board the train ONLY AT B and NOT AT A, even if you have been charged the fare from A to C, for the same reservation.
Even in a case where the original ticket was from A to C, with boarding at A, if the boarding point is subsequently changed to B, you forfeit your right to board the train at A. I hope I have been able to clarify the matter to your satisfaction.