We did change this property as well and it appears to work when you remove the course and add the course back to the curriculum. However, when the nightly batch runs it assigns the expired session again. We have found a work around since it will only assign the expired session to the users that did not complete the course. We have used the Bulk Unenroll Users to unenroll the users in the session that is expired and not complete. An easy way to create the import is to run the status report for incomplete/not attempted courses. You will need to change the import headers and remove the session title and only keep the ID of the session to unenroll the users but it gives you all of the data in the report. Not a fun work around and we are hoping with the upgrade to coursemill that this bug will get fixed.