I just received a voucher from a problem on a LHR-ORD flight for $400. It was good for domestic only. I called GS CS and they didn't hesitate to change it to an international one (had to send it in; received the new one a week later).

Considering your status and the fact that you were disserviced on an international flight, I would think your sucess in exchanging it would be approaching 100 percent.