code: public Subscription changeAmount( String subscriptionId, Integer amount ) {
      return this.changeAmount( new Subscription( subscriptionId ), amount );
    }
docs: Changes the amount of a subscription.<br>
  <br>
  The new amount is valid until the end of the subscription. If you want to set a temporary one-time amount use
  {@link SubscriptionService#changeAmountTemporary(String, Integer)}
  
  @param subscriptionId
  the Id of the subscription.
  @param amount
  the new amount.
  @return the updated subscription.