I've tried to upgrade an international Z fare for my past four flights. GS checkin at ORD, Narita RCC, Narita gate, ICN gate, etc. All the same answer: the computer screen literally will not let you enter the system for a change. Very nice NRT gate agent puzzled (as empty seats in first), called GS desk, no go.
This is a big deal for me, especially as folks I know on these flights had NRSA seats in FC. I've written a two page letter to USG, we'll see what happens. In short, my SWUs are worthless.