Minimum speed reversion is available in altitude hold and with G/S capture when the auto throttle is ON. In these cases the A/T can command a higher thrust setting, because it's ON. If it is off the aircraft will try to hold either level flight or glide slope at the expense of speed. The same goes for VNAV path when flying a level segment.
In other cases where the auto throttle is OFF, minimum speed reversion is still available since the autopilot is able to control the speed using pitch. If flying in vertical speed mode or VNAV (climb or descent) the autopilot will revert to Level Change.