The new Question Engine was the big change in Moodle 2.1. If you have a lot of Quiz attempts then you can use the Question Engine Upgrade Helper tool to upgrade the quiz (question) attempts in stages. If your quiz attempts were correctly upgraded (the question attempts data after the upgrade = the question attempts data before the upgrade) then you can ignore that "list already upgraded quizzes..." option. It's there for when you find problems with a converted question attempt and want to redo the conversion for that attempt to see if it fixes the problem.