Quizzes are complex beasts ...  

I've been asked if changing questions in a quiz will affect completion status for those who already took the course. 

As I understand it a user's grades are fixed for a quiz attempt whatever happens to the quiz later. Is that correct?

And I presume that the overall grade percentage is all that is considered for completion tracking so even changing the number of quiz questions or recalculating grades should not affect the requirement of a percentage quiz grade even if we were to unlock completion tracking.