At some point when lowering the voltage, the hardware error rate will start increasing but nothing else should happen. Just in case you manage to still trip some undocumented failsafe and the unit ends up in a reboot loop due to the test settings having been saved, you'll still always have the option to re-flash the sd card and return back to the default settings.

Note that it may take some minutes for the hardware error rate to stabilize so don't be in too much hurry when making voltage changes.