technically can, but depends on TTE, TTE may or may not wait.. may allot your berth / seat to next RAC pax in case of no show from your current boarding point..
Better to change your BP if you intended to board from GWL instead of AGC.. go to nearest PRS with tkt and original ID and give req. for change of BP..
BP change need to b done atleast 24 Hrs. prior to train's dep. from its originating station