I know that smart contracts are supposed to be immutable and that's the whole point, but to expect someone to implement a logic that never changes (no upgrade or no bugs) from day 1 is also unrealistic.
Therefore, I've been reading about several methods of working around this immutable state. A popular method seems to be using delegateCall with a Relay contract, but I'm struggling with how to actually use this method as I couldn't find any example.
Would someone be kind enough to look at that simple example I created, and tell me what I'm doing incorrectly?
https://gist.github.com/fabdarice/d513d620d9355312d085c7a68e6c6118

I know that smart contracts are supposed to be immutable and that's the whole point, but to expect someone to implement a logic that never changes (no upgrade or no bugs) from day 1 is also unrealistic.
Therefore, I've been reading about several methods of working around this immutable state. A popular method seems to be using delegateCall with a Relay contract, but I'm struggling with how to actually use this method as I couldn't find any example.
Would someone be kind enough to look at that simple example I created, and tell me what I'm doing incorrectly?
https://gist.github.com/fabdarice/d513d620d9355312d085c7a68e6c6118
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;
  }
}  

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
  }
}  

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() ;
})