Steps to replicate the issue:
- Create a course and enroll a student.
- Create an assignment with Grading method set to Simple direct grading.
- Login as a student and submit a submission.
- Login as a teacher and grade the submission.
- Edit the assignment and set Grading method to Marking guide or Rubric.
- Define Rubrics or Marking guide criteria.
- View all submissions.
- Confirm, that the submission is still in Graded state, but has no advance grading details.
It seems that Moodle should check if there any graded submissions that should be reset to ungraded when a grading method is changed.