Can I Upgrade/Downgrade my plan?

Yes, you can do this from your profile page

What you need to know:

Spark Shipping uses pro-rated billing. This means that if you decide to upgrade or downgrade your monthly account plan, you only pay for what was actually used and you can switch back and forth between plans as much as you like, even if only temporarily. 

How does an upgrade or downgrade affect my bill?

• The difference in a downgrade is given as an in-store-credit to your account; your monthly fees 
  will automatically deduct from this credit balance until it is exhausted
• Upgrades will reflect the price difference between your prior plan and current plan
• In all these situations charges are pro-rated right down to the day; you only pay for what you use; if you do not use the entire month, it is handled as a credit

Can't I just get a refund instead of a store credit if I change plans?

• No, anything not used in your plan will be given to you as a store credit; we do not give refunds under these circumstances

What happens if I close my Spark account and still have a credit balance?

• Unfortunately, we don't give credit card refunds on in-store-credit, so we would not be able to refund this credit balance back to your card

There is a SKU limit allowance with each subscription plan

• You will be charged an extra $10/m per additional 10K SKUs

See related article:

Keeping tabs on your SKU counts