I tried updated to JB from Viper 2.7.1 by attempting to go back to stock using RUU_ENDEAVOR_U_ICS_40_TMO_UK_1.28.110.7_R2_Radio_1.1204.103.14_release 256500_signed.exe & it didnt go to plan.
After I relocked the bootloader I couldnt get any further with the process & my phones battery ran flat...