In Moodle 2.4, 2.5 or 2.6. Go to Admin -> environment check, and check whether your site is ready for 2.7. There should be no sign of the new test since it is passing. In the database, change quiz_attempts.needsupgradetonewqe to 1 for one row. Now the environment check should tell you that you have problems (at the bottom). Now, in master Run the upgrade. Verify no errors. Look in the DB, and check that the question_session and question_states tables are gone, as is the quiz_attempts.needsupgradetonewqe column. Check that tool_qeupgradehelper is no longer listed in the admin menu. Check that its version number is no longer in the config_plugins table. Get a Moodle 2.0 backup including a quiz with user data. Restore that backup, and verify that all the quiz attmepts have been restored properly (compare the Attempt review in 2.0 and master versions).