Attack: if I change the genesis block.

[runpaint]

Each block contains a hash of the previous block header.  So you can't change any block without changing every block that comes after, which also eliminates the possibility of inserting a block in-between existing blocks.  That's what makes it a chain.

[dinofelis]

Even if clients didn't check the genesis block, you'd still have to make a chain with more proof of work than bitcoin

Yes, that's why I said, once I heard that it is the max PoW and not the max number of blocks that counts, that the check on the genesis block is actually moot.
If you can put in more PoW than the actual chain, you can just as well keep the original genesis block.