GX450 upgrade when power is not guaranteed

We are looking to upgrade the firmware on GX450s on a locomotive fleet. While we intend to ensure that the power is available during the entire upgrade operation, I need to account for a situation in which the modem loses power during the upgrade.

Will it brick the modem or will it simply revert to old firmware version and config?

Hi Pete,
Please make sure your device is not entering Low power mode or reset during the upgrading.
If there is any interruption during the upgrading, your device could be broken.
Please refer some notes in Software Configuration User Guide: