You need to find or create a quiz that has several attempts made by different students. Go to Quiz -> Results -> Grades. Click the Regrade all button. Verify it works with no errors, and the progress information looks like "Regrading attempt (1/3) - Attempt 1 by Student Two (id 426)", then at the end it will change to "Successfully regraded (1/1)". (You may see different numbers) Select some attempts using the Check boxes, and click the "Regrade selected attempts". Verify as before. Edit one of the qustions in the quiz to change what the right answer is. Click "Dry run a full regrade". Verify as before. You should not see that the scores for some students would change (old score crossed out, new score shown). If not, edit the question differently. Click "Regrade all attempts needing it". Verify as before.