Yes! But make sure to do so before the 1st of the month (renewal date) by logging into your account dashboard and clicking "Edit" under "Subscriptions".
When you upgrade/downgrade your subscription, it does not reflect immediately (you might have noticed we haven't charged you the difference). It changes on your next renewal date (you can find this under "Subscriptions" on your account dashboard). So if you already paid for the April Original Box on April 1st and you upgrade on April 18th to the Deluxe, you won't receive the Deluxe Box until May. If you want the upgrade immediately, just shoot us an email at firstname.lastname@example.org and we'll charge you the difference via Paypal.