Once you publish a quiz and make it available to your students, you can make changes to the quiz. Original quiz data is retained as part of any existing student submissions. However, students who have already opened or completed the quiz will not see any changes you make, which may affect their grades. These types of changes include adding new questions or deleting questions from the original quiz.

Additionally, if you change only the point value of the quiz, the student's submitted quiz will show the updated point value, but the current grade won't change in the Gradebook.

If you only need to make changes to existing quiz questions, after editing the quiz you can use quiz regrade to edit certain types of quizzes, or you can add fudge points to individual student quiz scores using SpeedGrader. You can also moderate the quiz to give additional attempts.