Hi Jessie,
If you reset the completion options then save, all completion information for the quiz will be regenerated according to the new settings. The edit quiz settings page will then say 'Completion options locked' with an 'Unlock completion options' button.
If you ticked require grade, the students who have already attempted the quiz and obtained a grade will show in the activity completion report as having completed the activity.
Please see the section 'What happens when you unlock' in the documentation Activity completion settings for further details.