What happens if I somehow manage to install older OS on Y HW

I was searching on how could I downgrade my revision Y calculator from OS 4.2 to OS 3.6, but it seems that my calculator doesn't get lower then that and nasgmr doesn't allow me to do that neither.
So I was wondering, what really happens if a system lower then then the supported by the hardware revision would do. Why it can't be done? Does the hardware really change a lot from a older revision to the Y revision?
Thank You
So I was wondering, what really happens if a system lower then then the supported by the hardware revision would do. Why it can't be done? Does the hardware really change a lot from a older revision to the Y revision?
Thank You