Changing the quiz setting like that will not work, because the behaviour being used is stored when you start the attempt.
If you could change the behaviour being used in the database, and then regrade, then the regrade would sort things out.
However, changing the behaviour in the database is not so easy. It is stored in both question_attempt.behaviour and question_usage.preferredbehaviour, in all the rows that related to a particular quiz attempt. http://docs.moodle.org/dev/Overview_of_the_Moodle_question_engine#Database_tables