Actually, it will go down automatically to any previously active subscription, which has been
upgraded to the expired one, provided that the previous subscription is still active.
e.g.:
Member registers for 1$ 5-years beginner plan
Then upgrades to $10 1-month premium
After expiry of premium, beginer plan is automatically re-activated, and re-subscription to premium is possible (if allowed by plan configs)
Otherwise if no upgrade occured, it will automatically revert to a free lifetime subscription, if free users are allowed and subscriptions don't need to be created for free lifetime subscriptions in general CBSubs settings.
That's how it's designed and already works (CBSubs 1.0.1)