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Instructors and course adminstrators can edit an existing quiz and it will not affect the student's scores for any attempts they have made for that quiz in the past. So their quiz score history will remain the same even if you totally change all of the questions and answers. Also if they have earned a passing score, their assignment will remain in "complete" status and they won't have to retake the updated quiz.
As an instrucor or course adminstrator, you can edit the existing quiz questions to change the wording or change the answer choices, however in order to keep the past score reports accurate you should make sure that you don't change which choice is the correct answer. So for example below, keep the correct choice as "A" even though you are changing the question and answer.
1 How many apples should you eat every day?
A one apple
B two apples
Correct choice: A
1. How many apples should you eat every week?
A seven apples
B two apples
Correct choice A
You can also remove questions and replace them with new ones. If you remove any quiz questions, when the students view their "score report", the removed question will no longer show up on the report. So if the question and choices are changing drastically, it may be better to delete the question and add a new one, just so the past score reports aren't confusing to students who took the old version of the quiz before it was updated.
The alternative would be adding a brand new quiz, then removing the old assignment and replacing it with the new quiz assignment. In that case, students who haven't yet earned their certificate will need to complete that quiz in order to do so. The benefit of this strategy is that the new updated quiz would likely be helpful for them and it would be good for them to know it's a new assignment and new quiz version.