You have it backwards.
The oldest 360 had the highest power requirements and can only use the brick that came with it and not the 175 and 150 watt ones. As they redesigned the chips in the console as they revised the system, it lowered the power requirements as the chips became more efficient. So they shipped power supplies with less capacity.
You can use the original 203 watt power supply on a 175 or 150 watt system since it has the necessary capacity. And you can use the 203 or 175 watt power supplies on the newest revision of the original 360, the 150 watt system.
But you cannot use the 175 or 150 watt power supplies on a 203 watt launch era console, and you cannot use a 150 watt power supply to power a 175 watt console. The plugs are designed to make this physically impossible.
That's why Satoshi_Matrix can use his launch model's power supply (203 watts) on his 2010 HDMI console (Likely 150 watts). But if he tried to use the 150 watt power supply that came with his new console on his launch era system, he'd find that it wouldn't even physically be able to be plugged into it since it doesn't have the capacity necessary to power it.