In generally, u do not have to do anything. Just board the train from SBC station. if tte ask anything- that where were u? I didnt see u? etc, then u may say that- i was in bathroom, I personally did it many times. Normally- TTE checks the vacant birth upto next 3-4 stations. Then he may allot that birth to others. U do not have make any application or change anything......just board the train.