Relay contract that contains only one state variable 'user_amounts' - Link first to Donation contract, then update to DonationNew to add the cancelDonation function.

app.js

// First, deploying Relay, then deploying Donation and retrieve Donation contract address in 'donation_contract_address'

// Then, linking Relay to my first version of my contract Donation 
Relay.deployed().then(function(contractInstance) {
   contractInstance.changeContract(donation_contract_address);
})

// Then, I want to call sendDonation from the Donation contract
// !!!!! I DON'T KNOW WHAT IS THE CORRECT WAY TO CALL THIS !!!!!!
Relay.deployed().then(function(contractInstance) {
   contractInstance.sendDonation(5) ;
})
// OR 
Relay.deployed().then(function(contractInstance) {
   contractInstance.currentVersion.delegateCall(sendDonation(5)) ;
})

// Now I want to update the Donation contract to add the cancelDonation function
// First I deploy the new contract DonationNew and retrieve it's address in 'donation_new_contract_address'
Relay.deployed().then(function(contractInstance) {
   contractInstance.changeContract(donation_new_contract_address);
})

// are the state variables still available from the old contract to the new one?

// Then if I want to call the new function : 
Relay.deployed().then(function(contractInstance) {
   contractInstance.cancelDonation() ;
})

Donation.sol

contract Donation {
  mapping (address => uint) user_amounts;    
  
  
  /* DOES THIS METHODS MODIFY user_amounts of the Relay contract ??? */
  function sendDonation(uint n) {
    user_amounts[msg.sender] = user_amounts[msg.sender] + n
  }
}

DonationNew.sol

contract DonationNew {
  mapping (address => uint) user_amounts;

  function sendDonation(uint n) {
    user_amounts[msg.sender] = user_amounts[msg.sender] + n
  }
  
  function cancelDonation() {
    user_amounts[msg.sender] = 0
  }
}

Relay.sol

contract Relay {
    address public currentVersion;
    address public owner;
    mapping (address => uint) user_amounts;
    

    modifier onlyOwner() {
        if (msg.sender != owner) {
            throw;
        }
        _
    }

    function Relay(address initAddr) {
        currentVersion = initAddr;
        owner = msg.sender; // this owner may be another contract with multisig, not a single contract owner
    }

    function changeContract(address newVersion) public
    onlyOwner()
    {
        currentVersion = newVersion;
    }

    function() {
        if(!currentVersion.delegatecall(msg.data)) throw;
    }
}