On Sunday, I traveled from NDLS to SC on this train. My ticket was for SL RAC 17, 18. When I checked the berth allotment for me, to my surprise I was upgraded at no additional cost to 3AC. Happily, I completed my journey, but I was curious to know how this upgradation actually takes place. For your info, the passengers who were originally alloted my berths in 3AC were upgraded to 2AC. The journey was good and the train reached SC before time. Please if someone can shed some light on the upgradation process and the criteria for upgradation. I do not remember having checked the column that says "consider for auto upgradation" when booking the ticket from irctc.