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You'll find this process much easier if you already know which airline(s) you might want to fly, as you can then search their website(s) for full fare and/or fully refundable tickets, which usually allow free date changes. Note that, depending on the airline, some tickets that are not fully refundable and/or not full fare may also allow date changes, so this is just a simple way of getting your search started.

If you're not sure which airline(s) to use, try searching the route on ITA Matrix or another online fare searcher to help you identify carriers whose routes and schedules appeal to you.

Finally, all things being relatively equal (price, schedule, etc.), you should also consider the various airlines' alliances and frequent flyer programs. You'll want to focus your wife's flying on partnered carriers as she will likely achieve status fairly quickly if flying this route several times a year in a premium cabin.

Good luck!