I tried to change the due/until dates on a Canvas quiz after I initially released it to my students. Although the new dates were correctly displayed in the Quizzes area, the system automatically submitted students' work on the original due date! I'm not sure if this is the case for students who had not yet started the quiz before I made the change, but it is certainly true of everyone who did.
I know that quiz *content* changes (questions/answers) are not seen by students who have already started it (which is a very significant limitation), but I gather now that this limitation also applies to changes to the dates, although this was not at all clear to me from the Canvas documentation.
I'd be very grateful for clarification on whether there are in fact limitations to quiz date changes, or if I misunderstood how to go about this correctly.