Another data point,
Want to change next sector on a DAS13. Booked flight is tonight. Want on next available flight (ITM-HND) approached JAL, they said no, contact QF. Even though JL has changed their own flights on non JL tickets on oneworld tickets before. -_-


This is getting quite annoying.
For tickets that are flexible, that, in theory and according to oneworld any oneworld member can change, the airlines are getting rather difficult about actually making changes.

In years past, I've had CX, JL, BA, QF, AA and AY all make changes to in progress bookings, to both their own flights and others, no matter what stock the ticket was on.
However, in the past 2 years or so, they've been much more difficult. Some agents have outright refused. Others have attempted to charge the reroute fee for changes that didn't require it.
This current trip, a few weeks ago, I asked to change a flight that was 3 weeks later. QF ticket, AA code, CX flight. I asked QF to change it as ticket issuer. They said to talk to CX as it was a CX flight. CX said to talk to AA as it was an AA code and AA said to talk to QF as it was an QF ticket.
Then today, something came up that made me need to spend the day in Tokyo instead of Osaka. Had been expecting an easy change. Show up at airport early, ask JL to change a JL flight. They say to go talk to QF. QF doesn't respond, contact travel agent. Spend 2 hours trying to get replies from QF or agent while watching as flights I could be on with open seats depart.
Travel agent has no luck changing the flight, can't see the availability that I can see on the airport screen in front of me. The JAL international connections counter reopened, and I spoke to them. They said to talk to QF, but by then QF had finally responded that they can only see waitlisted, they don't get live availability from JAL. That message was enough to get JL to open a seat and contact QF to make the change.
So, on a flight, 4 hours later then planned.

This has become much harder then it used to be, or needs to be.