Downgrading the level of your BJ's Perks Rewards Membership can be done only at the time your Membership comes up for renewal. Your earnings will accrue through your renewal date.
If you are renewing on BJs.com, you can downgrade by clicking on the Renew link, then clicking the "Remove Upgrade" link before adding your renewed Membership to the shopping cart. That will allow you to renew your Membership as an Inner Circle® Member rather than as a BJ's Perks™ Rewards Member.
Your Membership number will stay the same if you upgrade or downgrade.
If you have any further questions about changing the level of your Membership, please ask for assistance at the Member Services Desk in-Club.