I think the most obvious change was going from single flap to double flap....IMO, the double flap looks thicker and the chain is different....
This is really only relevant to Jumbos, and actually in my opinion Jumbos have seen the most changes over the decades. M/L classic flaps and square minis have maintained more consistencies over the seasons. They had East/West classic flaps for a bit, then they came out with the rectangle minis in more recent years. I've seen seasons where some minis had a double chain system like M/L's and jumbos. They definitely try some variations every now and again. But again the core design of what makes a bag a classic flap is maintained.