You have paid the fare for TBM-RMM , so ethically you can board from TBM. But if legally seen, then you are not allowed to board the train before VRI. So i would suggest you once board the train at TBM and meet TC. Tell him you "last moment change over" and i don't think he would mind that much. But you can be well trapped if IR would have allotted someone else your seat for MS-VRI stretch bcoz once you change your BP then railways release your seat for MS-VRI stretch as vacant. System can allot someone else to travel for TBM to VRI. But all depends on luck. Board the train at TBM and wait for TC.