I was not able to make that change. Apparently the change can only be made with the same airline in each leg.

additionally, my inbound flight was on ANA while the outbound was on EVA/Asiana as explained on my post, so I wasn’t able to make date changes online. I had to call and suffer through the 2 hour hold. But once I got through I was able to change the dummy inbound booking. So we are good to go!