Interestingly I had missed this major DL policy change to the negative in terms of actual changes in the scheduled timing (as opposed to aircraft type changes or flight number changes etc) - and which actually rolled out a year ago - yikes

It was only after a 2+ hour schedule change today on a 3-segment multi-country ticket in Delta One, I learned that new rules now only permit the schedule change policy to be applied if you either depart or arrive at least 6 hours or more from original schedule (International itineraries) and 3 hours for trips wholly within the US.

FYI for multi-city itineraries - the most restrictive policy applies to entire ticket (thus even a flight within the US only on a given date would have to be more than 6 hours retimed).

It's not just DL of course, many carriers are adopting customer unfriendly moves like this with much less flexibility for those impacted by schedule changes. In my case, the schedule change was such that I could now comfortably make a morning departure connection instead of a red-eye with several hour layover in ATL which was not an option when I originally booked... But DL now requires a change fee in addition to repricing of ticket. This may thus govern my strategy in future of booking flights early, I'd often book 8-9 months out and Sked changes not uncommon but this can now really disadvantage you when booking early.