I am a 2021 cx5 GTP owner, I bought a 2024 cmu from a 2024 signature wrecked on ebay. I swapped it out and plugged all the ports in (same ports/wire harness) but no display, the car is drivable. the cmu fan spins. i plugged back my orginal cmu and it took a while to boot back up.
I personally feel like the only way is to figure out what extra 24 parts needed to wake the cmu up . would need buy a fully wrecked 2024 cx5, and have a 2021 to test.
from what I seen, 21-23 all the same. I do have another 2023 cx5, and the cmu from my 21 boots straight up.
i also tried the 2024 usb C hub part, the connection is the same on 2021, plugged it in, lights up, but cmu says that the hub is not compadible directly on the screen, no poweroutput.