The most significant IOC rule change since 1968 is the delegation of the operations to the international sports federations. Whereas before the IOC ran everything, now FINA runs swimming & diving, IAAF run track & field, FIFA runs football (soccer), and so on.
This was a huge change because it also meant the international sports federations decided on eligibility. Thus far, every sport except boxing have allowed professionals to compete, which they largely did by 1992.
I'm an Olympics junkie.