It has been a while so things may have changed so you should double check with support@cgcookie.com. With that said, If you cancel just the subscription then it should revert to a free account. You will still have access to the forum and any free videos. As long as you don’t delete the account the data will still be there. So, make sure you are just canceling subscription and not deleting the account. They use to be separate buttons.
***Edit***
I just looked under account settings and they are still separate buttons. Cancel Subscription is under subscriptions and Delete Account is under Privacy. You should still double check with Support. Better to confirm from someone who works here.