When you upgraded to your new plan, you still had time left on your previous subscription. When that's the case, we don't charge you immediately when you upgrade. Instead we adjust your expiration date based on your new plan's price (described on https://www.zotero.org/support/storage_faq#how_can_i_change_my_current_storage_plan). That new expiration date is when the first charge at the new subscription level is due.
Upgrade Storage
When you upgraded to your new plan, you still had time left on your previous subscription. When that's the case, we don't charge you immediately when you upgrade. Instead we adjust your expiration date based on your new plan's price (described on https://www.zotero.org/support/storage_faq#how_can_i_change_my_current_storage_plan). That new expiration date is when the first charge at the new subscription level is due.