FARE RULES:
I pulled down the DL fare rules (these are BUD-SEA but they all appeared nearly identical). I posted them on a share. We can easily grab the AF and KL variants if others need those.

Q: This is a fully flexible fare with free changes. Does that mean I can change the flight dates as I want without paying anything extra?
A: No. The airline simply doesn't charge a change fee. But depending on the fare rules, you will still need to pay the difference to the new price.

Q: But I read that some people could change their flights for free...?
A: The rules for this fare (linked to above) specify that historical pricing shall be used if either at least one flight from the ticket has been taken or the outbound remains the same. In other words, in order to be able to change your flights for free, you must have flown at least one flight from your booking. Otherwise, you can only change the inbound. The outbound has to remain the same or you will be charged the fare difference.

Q: Can I change the rest of the itinerary after flying the first segment without any other charges or additional fare differences?
A: Yes this is possible according to the fare rules where after completing the first segment, you can change the rest of the flights for free (with the condition that the same fare class is available- I-class - in this case. According to the fare rules, after flying the FIRST segment (in the case here BUD-XXX) the historical fare is used. Possible to use the first stopover at AMS or CDG (see post #830).

Q: But some people could change their outbound flights for free!
A: They got lucky. It was probably a bug on the website/in the app. When no flight from the ticket has been flown, the fare rules do not allow changing the outbound without repricing.

Q: Ok, ok, so I can only change the inbound for free. How do I do that?
A: You search for availability for alternative flights on ITA Matrix using "F ..I2FFHU" in the extension codes. You must search for round trip flights as this is a round trip fare. Then you ignore the outbound result (as long as you haven't flown at least one flight from your ticket) and just try to rebook your inbound on the website/app.

Q: I have found availability on ITA Matrix, but the app/website doesn't show me those flights!
A: Keep trying. Sometimes you may have to serach in a different way (split up the searches, e.g. HNL-LAX and LAX-BUD), sometimes you just have to repeat the search several times. But that's no guarantee the desired results will turn up (and without repricing).

Q: It doesn't work. Can I just call the airline to make the changes?
A: Please don't unless you need to. Calling can increase the risk for all of us that they'll change their mind and disallow all further changes, or simply cancel everyone's tickets. However, as this fare has now been running for months and has seen many calls made about it, it is reasonable to expect that calling will not cause issues at this stage. As best practice, avoid attracting attention on the low price paid, and avoid making additional demands or tempting your luck. Remember you get to fly a J ticket for the cost of a Y fare.

Q: How many times can I make changes?
A: In theory, as often as you like, but you can't make more than two changes online/in the app. After that, you'd have to call, which we are trying to avoid.

Q: I've changed my mind. Can I get a refund?
A: Yes, the fare was fully refundable. Just contact the airline or fill out a refund form online.

Q: Due to Coronavirus, I need to extend my travel plan beyond the original expiry date. What can I do?
A: For Delta tickets (starting with 006) you can travel at any point until December 2022. Changes to flights within a 10 month window can be done at any point on the app, following the rules above. If you want to fly later than 10 months from now, you can call Delta and ask them to "park" your ticket. You will receive a ticket number that ties to an eCredit which can be used later to buy tickets. As long as you keep the original itinerary, or if you have flown at least a segment, you will not be charged a fare difference. Bear in mind that the eCredit may only be refundable if your original flight was cancelled (which is very likely in those day and times). Note that only Delta in the US are able to do this so do not call the EU numbers (use Skype to save money on international calls). According to user feedback: iMessage agents seem to also be able to do this, but twitter agents not.