Someone who actually knows what they are talking about will need to chime in, but I believe an appearance in a fall scrimmage/game against another school burns a year of eligibility. A couple of years ago, my son's team had a scrimmage/game like this. There were 2-3 kids on the bubble for being cut at the end of fall practice. Coach called them into his office before the game and gave them the option of sitting out in order to preserve a year of eligibility in case they later got cut and decided to transfer.
Again, someone will need to confirm or correct this understanding.