Changing the Boarding point is allowed till 24 hours prior to departure from the starting station.
Normally the TTE will wait till the next station, but by rule he can allot the berth to next eligible RAC passenger at any time if the passenger found as not joined.
Submit the requisition letter along with e ticket copy and ID proof at PRS counter.
It...
more... can be done any time during PRS hours till 24 hours prior to departure from the journey originating station.