I flew QR CPT-DOH-LHR-DOH-CPT in economy in 2023.
On each leg I was offered an upgrade fare via the QR app, which for the first 3 legs was around £1,000 each. I declined. Then on the last leg DOH - CPT, daytime flight of around 9.5 hrs...up came the upgrade offer....£400. No brainer! And it was a lovely A350 to boot.