Is it possible to upgrade the subscription plan?

Depends on the payment method

It is possible to do an upgrade when you’ve paid for the initial order with Stripe

PayPal does not allow upgrading

How to upgrade?

To do this, log in to My Account and go to Subscriptions

There, you will see an Upgrade or Downgrade button. Click it and you will be taken to the product page, where you should select the new subscription plan and proceed to checkout. You will have to pay just the difference in price between plans. 

The new API key will be sent by email and it will be also available in your account.