All Nippon Airways (ANA) has a fantastic breakdown on their website to help consumers quickly determine whether they can get a refund if their fare drops after purchase. I reprint it here in its entirety (complete with graphics) because it’s awesome and will really clarify all of your questions about when you can get a partial refund if your fare changes:
Applicable fares are the fares (including fuel surcharges) and other charges in effect on the date of issue and applicable on the date of commencement of the itinerary of your ticket.
Therefore, even if there is a fare increase or decrease, there will be no additional collection nor refund after you have completed your purchase.
(1) On August 10th, you paid JPY100,000 for your ticket for travel commencing on January 10th
(2) On October 3rd , there is a fare decrease to JPY80,000.
→No Additional Refund
(1) On August 10th, you paid JPY100,000 for your ticket for travel commencing on January 10th.
→No Additional Collection
However, if you have purchased your ticket with a fare where rebooking or rerouting is permitted, and you wish to change your booking or routing for the first international segment of your ticketed itinerary before commencement of travel, applicable fares(including fuel surcharges) and other charges shall be those in effect on the date the itinerary on the ticket is changed. Therefore, in the following cases, the fares to be applied will not be those applicable at the time of your purchase. In such cases, when there is a fare decrease or increase, the difference in fare(including fuel surcharges) and other charges will be collected or refunded at the time the ticket is changed.
NOW I get it! (Yes, this is why people hate airlines).