I have flown Premium economy with BA/Qantas Oz to UK via BKK.
At BKK I took my time to return to the gate and surprised by bump up to business class upgrade. My previous seat was needed to cover overbooking.
So in middle class aircraft and frequent flyer membership and tardy reboarding may have helped.
Flying single gotta help as well. Sometimes at desk check-in I have been offered discounted upgrades! Hard to do with web check-in.