Issue when Editing/Updating row values of jqxGrid bound with a Knockout observable array, replace observable object edited with a javascript object

How prevent this behavior of changing object in observableArray when user modified or edit a row in jqxGrid. I have a knockout observable array of Users, where each User have all is properties as observables,then I bound that observable array to a jqxGrid (from jqWidgets). And when I updated in the grid a User value, then in the observable array that item is replaced with an object containing the values of the observables of this User.

Below is my code:

HTML Section:

<div class="container">
    <div id="jqxGrid"
        data-bind="jqxGrid: {
        source: Users,
        editable: true,
        width: '100%',
        selectionmode: 'singlecell',
        columns: [
            { text: 'IsChecked', columntype: 'checkbox', dataField: 'IsChecked', width: '10%' },
            { text: 'FirstName', dataField: 'FirstName', width: '40%', editable: false, sortable: true },
            { text: 'LastName', dataField: 'LastName', width: '40%', editable: true, sortable: true }
        ]
    }">
    </div>
</div>

Javascript code: (Creation of Users array, ViewModel initialization, ko.applyBindings invocation.)

function UserModel (user) {

    var self = this;

    self.IsChecked = ko.observable(false); // later I will use this proerty in other place
    self.Username = ko.observable(user.Username);
    self.FirstName = ko.observable(user.FirstName);
    self.LastName = ko.observable(user.LastName);

    // I want to put a isDirty property here
}

var initialUsers = [
    new UserModel({ Username: 'JohnMicrosoft', FirstName: 'John', LastName: 'Microsoft' }),
    new UserModel({ Username: 'JohnGoogle', FirstName: 'John', LastName: 'Google' }),
    new UserModel({ Username: 'JohnApple', FirstName: 'John', LastName: 'Apple' }),

];

var viewModel = null;
$(document).ready(function () {

    viewModel = new UserViewModel(initialUsers);
    ko.applyBindings(viewModel);

});

function UserViewModel(initialUsers) {
    var self = this;

    self.Users = ko.observableArray(initialUsers);
};

This images show how the values changes after I click in the first row in IsChecked checkbox.

Result of clicking IsChecked checbox of first row in jqxGrid

The problem is that I want to add some functions to UserModel object and I want to keep the observables because I want to use them in other part of my page. I have een working in this for days and dont know what to do any help will be appreciated. Thanks.