Subscriptions & Billing
Under the Billing link, you can see your current subscription plan.

You can change your plan at any time, and will receive a pro-rated refund for any previous subscription.
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Please note that you cannot downgrade your plan below current usage (eg if you've already used 3000 calls for the month, you cannot downgrade to a plan that only offers 1000 calls - not until the next month when your usage resets to 0).