As a point since QR has been mentioned: I've tried with Qatar at DOH (AA issued RTW ticket) on an AONE ticket that was in "D"/business for DOH-LHR (they never released a third seat into A, even though the plane was at F6A0, with me taking the other 2 seats, 24 hours before check-in). After about 30 wasted minutes of various combinations of Cash/Avios attempts they admitted defeat, despite various calls and supervisors being involved. The ticketing desk's message on cash was that they couldn't upgrade a non-QR issued ticket, but it did feel like their systems weren't cut out for this.