private void OnChangeUserDefinedToNotesRequested(
            IUserFilledSudokuBox userFilledSudokuBox)
        {
            if (userFilledSudokuBox == null)
            {
                return;
            }

            var foundViewModel = FindViewModel(userFilledSudokuBox.Coordinate, userFilledSudokuBox.ParentCoordinate) as UserFilledSudokuBoxViewModel;

            if (foundViewModel == null)
            {
                return;
            }

            // Replace user defined view model with predefined view model:
            var viewModelIndex = mSudokuBoxViewModels.IndexOf(foundViewModel);

            var numbersInSameGrid =
                GetAllNumbers((userFilledSudokuBox.ParentCoordinate, userFilledSudokuBox.Coordinate));

            var remainingNumbers =
                mModelsFactoryService.SudokuNumbers().Except(numbersInSameGrid);

            var numbersInOtherGrids =
                mSudokuService.GetExistentNumbers(
                    (userFilledSudokuBox.ParentCoordinate, userFilledSudokuBox.Coordinate));

            if (numbersInOtherGrids != null)
            {
                remainingNumbers = remainingNumbers.Except(numbersInOtherGrids);
            }

            var newNoteViewModel =
                new NoteSudokuBoxViewModel(
                    mModelsFactoryService.GetNoteSudokuBox(
                        userFilledSudokuBox.Coordinate,
                        userFilledSudokuBox.ParentCoordinate,
                        new List <SudokuBoxNumbers>(remainingNumbers)),
                    mSudokuService);

            mSudokuBoxViewModels.RemoveAt(viewModelIndex);
            mSudokuBoxViewModels.Insert(viewModelIndex, newNoteViewModel);

            RefreshValues();
            SelectSingleAndUniqueNoteNumbers(GetAllNoteViewModels());
        }