FAQ: Fare Class Definitions (including which fares are upgradeable)

As IndustrialPatent wrote in his thread

"using miles to upgrade – use the award charts to find appropriate mileage amounts
persons without WorldPerks Elite status:
Any fare other than T or K can be upgraded; upgrades from Y come from “P” and upgrades from discounted fares come from “F.”

persons with WorldPerks Elite status:
Y fares will receive a complimentary upgrade if there is “P” availability; all others can be upgraded with miles."