Unless there are other minor differences on type, and assuming AP is on:
1. FLCH in B744 will eventually result in idle power descend.
2. The pitch controls the speed and the AT is at IDLE then HOLD.
3. AT in HOLD mode means the pilot can adjust the throttles freely.
Therefore increasing the thrust will add power to the descend and the AP will react by reducing the pitch and thus the rate of descend.
RC 123