how to change $scope variable value and test angularjs jasmine

I have the First case when product id should be undefined.

var $stateParamsStub = {
    productId: undefined
}

In my controller, I am checking for isUpdate true or false. If productId is undefined it means isUpdate should be false in another case it should be true

$scope.isUpdate = stateParams.scheduleId ? true : false;

My Test File

$injector.invoke(myController, this, {
    $scope: $scope,
    $stateParams: $stateParamsStub    
});

My first test case, which is working as expected because I set stateParams.scheduleId to undefined while injecting

it('should check update as false', function () {
    expect($scope.isUpdate).toBeFalsy();
});

My case 2, which is failing

it('should check update as false', function () {
    $stateParamsStub = {
        productId: "86C07C05-41FB-41E9"
    }
    $scope.$digest();
    expect($scope.isUpdate).toBeTruthy();
});

The output of the second case: Expected false to be truthy.

I know before every "it" before each gets executed where It is setting the default value which I assigned earlier. How to change that value for another test ? so it takes an updated value?