code: public Subscription changeOfferKeepCaptureDateAndRefund( String subscription, Offer offer ) {
      return changeOfferKeepCaptureDateAndRefund( new Subscription( subscription ), offer );
    }
docs: Change the offer of a subscription.<br>
  <br>
  The plan will be changed immediately.The next_capture_at date will remain unchanged. A refund will be given if
  due.<br>
  If the new amount is higher than the old one, there will be no additional charge. The next charge date will not
  change. If the new amount is less then the old one, a refund happens. The next charge date will not change.<br>
  <strong>IMPORTANT</strong><br>
  Permitted up only until one day (24 hours) before the next charge date.<br>
  
  @param subscription the subscription
  @param offer the new offer
  @return the updated subscription