Both Premium Membership and B&N Rewards earn stamps for eligible purchases made on BN.com, in Barnes & Noble stores, and on the Barnes & Noble App.
If a Premium Membership expires, it will automatically convert to a free B&N Rewards account. It can be converted back to Premium Membership status at any time by paying the $39.99 fee for an additional year of Premium Membership.
When changing programs, any stamps and rewards that are on the account at the time of the change will be retained. This applies both to upgrading to Premium or converting to Rewards. Stamps and rewards are never lost due to a level change.