Originally Posted by
Often1
UA tried this for a bit during 2018. I am not sure if it is still using it. Basically offered a free change on inflexible tickets for flights that were not merely overbooked but solidly at risk of an oversale. Reportedly not much uptake. Possibly there aren't that many people who have real flexibility any longer and flights are routinely full so the alternative may be a ways off.
I’m sure they’d get more uptake if the change were not just free, but compensated, even to the tune of $50 or so. That said, the high load factor on so many flights may mean that there just aren’t enough low-demand possibilities to switch onto.