When in HOLD and the speed reaches the MCP speed automatically changes in THR to maintain the selected speed.
No it absolutely does not.
IF auto-throttle wake-up engages, it puts the A/T into SPD mode, and either targets a speed just outside the low speed amber band if the selected speed is INSIDE the amber band, OR targets the bugged speed if the bug is OUTSIDE the amber band.
Correct ... when in Flight Envelope Protection (stall protection) mode the Autothrottle goes in SPD and on the FMA you'll see SPD.
In a normal descent when you select FLCH the FMA changes in
FLCH SPD and the Autothrottle will change in either
THR,
IDLE, or
HOLD. In FLCH the airplane will try to maintain the selected speed in MCP with the pitch attitude. The FMA will change into
SPD when you reach and intermediate altitude but you are not in FLCH anymore since the Pitch changes into
ALT