Default Revert back with Nandroid

I haven't frequented the forum in a long time but recently flashed JMX2 with the new 4.3 bootloader. From my understanding, there is no way of reverting the bootloader back to 4.1.2 or whatever it was before. My question is, would it screw up my phone if I didn't first flash back to a stock 4.1.2 firmware through odin before attempting to flash my back up nandroid? Is it necessary to flash a stock firmware to roll back to a previous version?