When a product has active subscribers, you can’t change its shipping date or variants. This keeps subscribers from being charged or sent something they didn’t agree to.
You can change its price. When you save, Cratejoy asks who pays the new price: new subscribers only, or existing subscribers too, through a change you schedule. See Changing Product Price.
To change the shipping date or variants, first create a new product with the settings you want.
You'll want to make sure that your shipping date, billing date, variants, and all other areas are configured to your liking.
After confirming the new product is set-up properly, you will need to move those active subscribers over to the new product.
To move subscribers to the new product, filter Subscriptions to them, then click Change Product Or Price… in the Action menu. See Changing the Product or Price for Many Subscribers.