Flexible flight?

You do know how airline tickets are priced in general. Only the discount tickets place restrictions on changing departure dates. If it is important to have a flexible return then buy a full fare ticket that allows for changes. All the airlines sell them. Truthfully the three days would not make that much difference to me. If it was a week that would be a different matter.

Typical change fee for a 'non-refundable ticket' is about $200, and on top of that you'll pay the fare difference, which can be anywhere between $0-$500+ depending on which fare classes are and are not sold out on a given flight. I'd guess it would be $400 or so 'all-in' to change the return flight at the last minute.

Note that this is still significantly cheaper than booking a 'fully refundable' ticket, which can run several thousand for international economy. Do you have any Delta Sky Pesos? They now allow one way reward flights, and you can apparently change them without penalty/fee until 72 hours before departure if there is similar award availability on your change date. I can see 30K miles and about $300 on fees options for both July 7 and July 10 currently. (Though there is no guarantee that the 30K on July 10 will still be there in mid-June)

Hi Alex, I thought you were required to show a return ticket when you arrived in Europe. Maybe someone can comment on whether or not that's true. Maybe it's different flying into London?

Nobody has ever asked to see my return ticket. Just my passport.

You can be asked your travel plans. Never happened to me but I am old and gray and probably don't look like I will hang out in Europe for six month. I have been asked where I am staying and for how long. On the other hand, my son and now DIL have been asked twice. Once they had to produce an flight confirmation since you don't have paper tickets anymore.

A border agent did ask to see my return ticket on my last arrival into London, which was only for a 3-night stay at the end of a 6-week trip. What I had handy to show him was not the ticket itself, but some other itinerary. One-way tickets are often more expensive than roundtrip, so although you only need to book another one-way, you might end up booking a roundtrip, and "accidentally missing" the second leg of it. Since you would intend to use the first leg of that, the exchange restrictions might be different than they are for the second leg. Much as I'd rather have 24 days in Europe than 21, I think it makes sense to proceed with the shorter plan.