Reading the previous version storage

SeparateStorages.js

mapping (address => uint256) private _balances;

// previous version of a token smart contract
ERC20 private _previous;

// flag indicates that migration of certain user balance was performed
mapping (address => bool) private _migrated;

function balanceOf(address owner) public view returns (uint256) {
    return _migrated[owner] ? _balances[owner] : _previous.balanceOf(owner);
}

function setBalance(address owner, uint256 new_balance) private {
    _balances[owner] = new_balance;
    if (!_migrated[owner])
        _migrated[owner] = true;
}