Modify jQuery plugin function externally

I’m working with iThemes Exchange for WordPress and I’m trying to modify the javascript file that controls certain aspects of the iThemes Exchange shopping cart checkout process.

The problem is I can’t modify the plugin itself because any changes will be overwritten when the client updates their plugins. So I’m trying to modify the javascript plugin’s updateStates() function so that the country label is automatically set to “United States”.

I’ve tried modifying the plugin using jQuery.fn.itCountryStatesSync which is where the plugin is, but I must be doing something wrong. If it helps I’ll include the plugin’s javascript file.

/**
 * jQuery Country States Sync Plugin by iThemes
 * Plugin framework via <http://coding.smashingmagazine.com/2011/10/11/essential-jquery-plugin-patterns/>
*/
;(function ( $, window, document, undefined ) {
    // Create the defaults once
    var pluginName = 'itCountryStatesSync',
        defaults   = {
            stateWrapper      : '.it-exchange-state',
            stateFieldID      : '#it-exchange-address-state',
            action            : 'ite-country-states-update',
            templatePart      : '',
            clearTextValue    : false,
            ajaxUrl           : itExchangeAjaxCountryStatesAjaxURL,
            adminPrefix       : false,
            autoCompleteState : false,
            autoFocusState    : false
        };

    // The actual plugin constructor
    function Plugin( countrySelectElement, options ) {

        // Set element
        this.element = countrySelectElement;

        // jQuery has an extend method that merges the
        // contents of two or more objects, storing the
        // result in the first object. The first object
        // is generally empty because we don't want to alter
        // the default options for future instances of the plugin
        this.options = $.extend( {}, defaults, options) ;

        // Country Field ID
        this.options.countryFieldID = '#' + $(this.element).attr('id');

        this._defaults = defaults;
        this._name     = pluginName;

        this.init();
    }

    Plugin.prototype.init = function () {
        // Place initialization logic here
        // You already have access to the DOM element and
        // the options via the instance, e.g. this.element
        // and this.options

        $(this.element).on('change', this.options, this.updateStates );
    };

    Plugin.prototype.updateStates = function( event ) {

        var iteCountryStatesSyncPostData = {};

        iteCountryStatesSyncPostData.ite_base_country_ajax  = $(event.data.countryFieldID).val();
        iteCountryStatesSyncPostData.ite_base_state_ajax    = $(event.data.stateFieldID).val();
        iteCountryStatesSyncPostData.ite_state_name_ajax    = $(event.data.stateFieldID).attr('name');
        iteCountryStatesSyncPostData.ite_action_ajax        = event.data.action;
        iteCountryStatesSyncPostData.ite_template_part_ajax = event.data.templatePart;
        iteCountryStatesSyncPostData.ite_admin_prefix_ajax  = event.data.adminPrefix;
        iteCountryStatesSyncPostData.ite_clearTextValue     = event.data.clearTextValue;

        $.post(event.data.ajaxUrl, iteCountryStatesSyncPostData, function(response) {
            if (response) {
                var parentForm = $(event.data.countryFieldID).closest('form');
                $(event.data.stateWrapper, parentForm).html(response);
                if (event.data.autoCompleteState)
                    $(event.data.stateFieldID).filter('select').selectToAutocomplete();
                if (event.data.autoFocusState) {
                    if ( $(event.data.stateWrapper).children('.ui-autocomplete-input').length !== 0 ) {
                        $(event.data.stateWrapper).children('.ui-autocomplete-input').focus();
                    } else {
                        $(event.data.stateFieldID).focus();
                    }
                }
            }
        });
    }

    // A really lightweight plugin wrapper around the constructor,
    // preventing against multiple instantiations
    $.fn[pluginName] = function ( options ) {
        return this.each(function () {
            if (!$.data(this, 'plugin_' + pluginName)) {
                $.data(this, 'plugin_' + pluginName,
                new Plugin( this, options ));
            }
        });
    }

})( jQuery, window, document );