Originally Posted by 21A

Just to update a little on what I learned here... It was originally a three-segment itinerary (double connection).
- in its original state no alternate flights at all were offered by web/app
- called and was able to move up the first leg by several hours, thus creating a stopover of ~12 hours in connecting city 1
Now the two parts of the trip seem to have separated for check-in purposes (checking in the first leg doesn't give boarding passes for the downward ones). In check-in for leg two, I get:
- options connecting to an earlier CCC-DDD (including more than one BBB-CCC for each)
- earlier BBB-CCC options connecting to the same CCC-DDD that's booked, up to a maximum connection of four hours.

To summarize,
- changing an earlier flight is no problem even if the second flight doesn't change,
- three-leg itineraries are too complex to do anything with without calling,
- the check-in automation appears to go by the 4 hour connection rule, and phone agents appear to have more latitude.

I should note also that none of the flights have been taken yet. I would imagine certain types of changes would only work after flying the first leg (e.g. move only the second leg later).