Ext.override(Ext.form.Field, {
    setFieldLabel: function(text) {
        if (this.getXType() == 'combo') {
            var label = this.getEl().up('.x-form-item').first('label.x-form-item-label')
        } else {
            var label = this.el.findParent('div.x-form-item', 3, true).first('label.x-form-item-label');
        }

        label.update(text);
    }
});