code: public Subscription changeAmount( Subscription subscription, Integer amount, String currency, Interval.PeriodWithChargeDay interval ) {
      return changeAmount( subscription, amount, 1, currency, interval );
    }
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 subscription the subscription.
  @param amount the new amount.
  @param currency optionally, a new currency or <code>null</code>.
  @param interval optionally, a new interval or <code>null</code>.
  @return the updated subscription.