Kindly explain your full query. The whole ticket is on defence warrant or just the Bju-Gwl mail part. If the whole ticket is on warrant then the first ticket would be your return journey ticket or onward journeu ticket too, if you cancel that the other ticket will be camcelled too. And the journey can be postponed or preponed. One thing also to note here, this is allowed only once. If only the Bju-Gwl mail ticket is on warrant, then there is no problem im changing it. For other queries kindly contact your colleagues or PRS.