Dont make fancy rules urself....Yesterday I've confirmed from PRS YPR,you can change the B.P. till chart preparation,provided that if you havnt changed it earlier...btw,the time limit for change of B.P.is given only for passenger awareness,so that they should not come @ last point,when the charts are already prepared....Even you may have noticed most of the PRS now-a-days preparing the charts for morning train in Morning itself,this is just to facilitate the last min cancellations & changing Boarding Pt...Rly doing their best so that passengers get the best service out of it...The 24 hrs was never a rule...In the System,it can be changed any time before chart preparation...!!!