Back in December Delta introduced the ability to upgrade with miles any time — from purchase leading up to travel. Your miles are worth only about a penny apiece and they charge you the difference in fare between what you bought and the price of the more premium option.
At the time, and leading up to when this was launched, I suggested that this could replace traditional mileage awards — where there are capacity controls, the airline makes seats available for upgrade they don’t expect to sell, and you pay a fixed (discounted) mileage price for those seats.