Look at it this way: It would be approximately as hard to change the rules of a mature bitcoin network as it would be to change say, the rules of the web (perhaps adding something easier to implement than <video>; more of a server-side change like say, TCP fast start) ...

But to make the change stick in the bitcoin network, the change would have to be made to all peers, network-wide, in the time it takes to calculate a few blocks. Otherwise the fork doesn't succeed.