Copy file using javascript client object model - new version

I have a solution that adds a custom button to the ribbon in SharePoint 2010 and am using the javascript client object model behind the scenes to copy a file from Library A to Library B when the button is pressed. The copy works fine but instead of adding the file as a new version in Library B (if the file exists there already), the file in Library B is just overwritten and the version remains at v1.0.

I have:

• versioning turned on in Library B
• the javascript performs a check-out of the file in Library B and then performs the copy

Is there something I'm missing? Has anybody accomplished this before? Is this not possible via the Javascript CSOM in SharePoint 2010?

Code:

var _ctx;
var _sourceFile;
var _destinationlibUrl;
var _destinationFile;

// LibraryA is the name of the 'source' Document Library
// LibraryB is the name of the 'destination' Document Library

function CopyFile() {

    // Get the current client context of the client-side object model (CSOM) runtime.
    _ctx = new SP.ClientContext.get_current();

    // Get the Web site that is associated with the client context.
    this.web = _ctx.get_web();
    _ctx.load(this.web);

    // Returns the list with the specified title from the collection.
    this.sourceList = this.web.get_lists().getByTitle('LibraryA');
    _ctx.load(this.sourceList);

    // Get the list items being selected.
    var selectedDocuments = SP.ListOperation.Selection.getSelectedItems(_ctx);
    this.currentItem = sourceList.getItemById(selectedDocuments[0].id);
    _ctx.load(this.currentItem);

    // Get the file that is represented by the item from a document library.
    _sourceFile = this.currentItem.get_file();
    _ctx.load(_sourceFile);

    _ctx.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded),
                           Function.createDelegate(this, this.onQueryFailed));
}

// Delegate that is called when the query completes successfully.
function onQuerySucceeded(sender, args) {
    if (_sourceFile != null) {
        _destinationlibUrl = web.get_serverRelativeUrl() +
                            '/LibraryB/' + 
                            _sourceFile.get_name();

        // Get hold of the file in the destination library.
        _destinationFile = web.getFileByServerRelativeUrl(_destinationlibUrl);
        _ctx.load(_destinationFile);

        _ctx.executeQueryAsync(Function.createDelegate(this, this.onLoadSucceeded),
                               Function.createDelegate(this, this.onQueryFailed));
    }
}

// Delegate that is called when the destination file checkout completes successfully.
function onLoadSucceeded(sender, args) {
    if (_destinationFile.get_exists() == true) {
        _destinationFile.checkOut();
    }

    _sourceFile.copyTo(_destinationlibUrl, true);
    notifyId = SP.UI.Notify.addNotification('Copying file... ' +
                                            _sourceFile.get_serverRelativeUrl() + 
                                            ' to ' + _destinationlibUrl, true);

    _ctx.executeQueryAsync(
        function (sender, args) {
            SP.UI.Notify.removeNotification(notifyId);
            SP.UI.Notify.addNotification('File copied successfully', false);

            //if (_destinationFile.get_exists() == true) {
            //_destinationFile.checkIn('Document Check-In',SP.CheckinType.majorCheckIn);
            //}

            // Redirect to Library B when complete
            window.location = web.get_serverRelativeUrl() + '/LibraryB/';
        },
        function (sender, args) {
            SP.UI.Notify.addNotification('Error copying file', false);
            SP.UI.Notify.removeNotification(notifyId);
            alert('Error occured: ' + args.get_message());
        }
    );
}

// Delegate that is called when server operation is completed with errors.
function onQueryFailed(sender, args) {
    alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
}