Other nodes don't accept your blocks if you mined/solved at a wrong difficulty (I hope) 
True so don't do that. Disconnect from the network and generate your own blockchain while working the difficulty down by reducing the hashpower in your network. Then when it's adjusted to a very low difficulty throw all your hashpower at that blockchain. If the difficulty increases far slower than it decreases you could well outpace the main chain. Then just connect to the live network again and everyone's chains get dumped in favor of your longer one and you have a shedload of coins. And all transactions made since you initially disconnected just got revoked.
The more hashpower you have the easier this attack is but it should work with just a small fraction of the total network hashpower. It all depends how asymmetric the difficulty adjustments are.
