by skoehler » Thu Nov 01, 2012 12:07 am
The ARM CPU of the NXT includes the so-called SAM-BA in-system-programmer firmware. If you push the reset button of the NXT for 4 seconds, this firmware is active and allows to reprogram the Flash memory of the NXT via USB. In other words: no matter how broken the firmware you tried to flash to the NXT is, you can always revert back to the SAM-BA firmware, which allows you to reprogram the brick via USB.
So yes, it's safe to switch between firmwares. Andy's answer seems to be more related to the write cycles of the Flash memory.