When an instructor modifies a quiz question in a Blueprint course, then sync with an actual course, the quiz in the actual course will be updated with the changes. If a student already took the quiz, Canvas will not do a regrade. From testing, regrade is only triggered per question in the actual course. This means the instructor will need to go to the actual course, and select the correct answer again for each question that needs to be regraded.
Suggestion: If an instructor sync changes they made to quiz questions in a Blueprint course with an actual course, there should be an option in the actual course quiz's settings to allow regrading the whole or indicate which question has been modified via a Blueprint.