You can upgrade, downgrade, or purchase new plans at any time. When you upgrade your plan, you will be charged for your new plan immediately and will be credited with a prorated amount for the previous plan. In contrast, the downgraded plan will only take effect for the next billing cycle. Namely, the system must finish the current subscription before switching to the new downgraded plan.

Moreover, there are no refunds or credits for downgrades.

For more info regarding subscription change, please refer to the support articles attached below.
How can I remove certain marketplaces from my current subscription?
How do I adjust my subscription plan? How will you bill me?