Fare difference when changing a return flight midway through the trip

I have a roundtrip ticket from the US > Japan > US with ANA. Their "flex special" fare allows flight changes for free + any fare difference.

My question is, how will the fare difference be calculated if I have already flown the first leg from the US to Japan? Will it be compared to a "historical" round-trip price with the new return date, or to a one-way ticket from Japan > US?

I'm concerned about this because the roundtrip ticket was about \$1700, but a one-way ticket with ANA from Japan > US costs about \$5000 (which is absurd, but that's another topic).

1 Answer

Fare difference calculations generally work by comparing fare classes. If you have (say) a class Q fare, and there's a free Q seat that matches your existing fare conditions (minimum stay etc), there's no fare difference. If all the Qs are gone but there's a K seat, you're charged the difference between the Q seat you paid for and the K seat you're "upgrading" into. These prices are on a per-segment (flight) basis for a return trip, so the crazy one-way prices you're seeing for a brand new ticket (which are likely full fare/Y fares) are irrelevant.

• It's a great explanation. Nov 3 at 11:43
• If only it was that simple... There's countless situations where that might not be the case. (the most obvious is changing the day of the week travel is occurring on - the 'Q' seat price can vary depending on that, so the price will change. There's also things like minimum/maximum stay/etc rules)
– Doc
Nov 3 at 17:20
• @Doc Correct, like all things airfare pricing the details are hellishly complicated, but the OP was asking if the difference is computed against an entirely new one-way ticket and that's manifestly not the case. Nov 3 at 18:40