Overriding datefield format in ExtJs 4 according to different conditions

Question

I want to override the datefield format depending on the country I am coming in from. I have written the following function, and then used the Ext.Loader.loadScript to apply this.But this is not overriding the datefield format. I am implementing this in ExtJS 4.Can someone suggest me how I can override the datefiled format. Thank you :)

function fnc() {
                    switch (params.country) {
                    case 'USA':
                        Ext.override(Ext.form.DateField, {
                            format : 'm/d/Y'
                        });

                    Ext.override(Ext.grid.DateColumn, {
                        format : 'm/d/Y'
                    });

                    Ext.override(Ext.picker.Date, {
                        format : 'm/d/Y'
                    });
                    break;

                case 'MEX':
                    Ext.override(Ext.form.DateField, {
                        format : 'd/m/Y'
                    });

                    Ext.override(Ext.grid.DateColumn, {
                        format : 'd/m/Y'
                    });

                    Ext.override(Ext.picker.Date, {
                        format : 'd/m/Y'
                    });

                    break;

                default:
                    Ext.override(Ext.form.DateField, {
                        format : 'm/d/Y'
                    });

                    Ext.override(Ext.grid.DateColumn, {
                        format : 'm/d/Y'
                    });

                    Ext.override(Ext.picker.Date, {
                        format : 'm/d/Y'
                    });
                    break;

            };
        }

Answer 1

Its working now, Added the following in the launch function in app.js at the start,

         Ext.Loader.loadScript({url:URL,onLoad:function(){
             debugger;

             var dateObj = {};
             dateObj.format = "";

        switch(country) {
            case "USA":
                dateObj.format = "m/d/Y";
            break;

            case "MEX":
                dateObj.format = "d/m/Y";
            break;

            default: // USA, etc. 
                dateObj.format = "m/d/Y";
        }
        Ext.override(Ext.form.field.Date, dateObj);      
         }
         });

And dont specify any format in the field definition,