upgradeWorkerStep2.sol

pragma solidity ^0.4.18;

contract Worker1 {
  uint public n;
  address public sender;

  function setN(uint _n) public {
    n = _n;
    sender = msg.sender;
  }
}


contract Worker2 {
  uint public n;
  address public sender;

  function setN2(uint _n) public {
    n = _n * 2;
    sender = msg.sender;
  }
}

contract Factory {
    
    address worker;
    
    function updateWorker(address _w) public {
        worker = _w;
    }
    
    function getWorker() view public returns (address) {
        return worker;
    }
    
    function doWork(uint _val, string fnName) public {
        worker.call(bytes4(keccak256(fnName)), _val);
    }
}