A (malicious) person who controls a majority of the hashing power can change the rules. Central to bitcoin is the idea that the longest chain is the valid one. If you control 51% of the network, you can create a chain that looks however you want it to look, and then publish that chain. Because your chain is longer than the correct one, it will be accepted, and your changes will be accepted. Using this, you can modify the chain to be what it would look like according to your rules.

Of course, this is only possible becuase your chain looks as if it was following the rules. If you have a non malicious computational majority, than any change in the rules would make an incompatible block-chain and a hardfork.

So how hard would it be to get 51% control of the network?