When your plan runs out - you can simply upgrade your current subscription package to the next tier. Keep in mind, that there are two different limit types that are important considering which subscription package you should choose - Order count and Inventory count.
If you reach the Order limit, you will not be able to ship through Multiorders and new orders will not come.
If you reach the Inventory count limit, you will not be able to upload any other products and update them.
Additionally, once you decide to move to another subscription tier, you have to pay the full price for the new subscription and not the difference. Once you do, the order limit starts counting from the subscription purchase.
If you have any further questions about subscriptions, please contact our customer support.