A few months ago I arrived at the airport on a paid ticket TLV-HKG where I'd asked my TA to use my points to upgrade me as I didn't want to risk not getting the Last Minute Upgrade. She screwed up and I was unhappy. One of the agents asked if I had a voucher. So it seemed that if I'd had a voucher I could have been upgraded but since I didn't have one I had no chance. In the end C was overbooked anyway.