“US carriers give away upgrade vouchers to their highest tiers, which can be used for paid seats in certain fare classes,” they wrote.
“They aren’t guaranteed, but they’re the best way to push the odds in your favour.”
This revelation was seconded by a passenger who had experienced this for themselves.
They explained: “Your chance of getting upgraded is based on some secret voodoo magic behind the scenes but likely there will be a correlation between the fare you bought (fully flexible refundable ticket vs. ultra-discount nonrefundable ticket) and your frequent flyer status with the carrier, and probably the time you checked in for your flight.
source: express.co.uk