Two years ago my wife and her father were traveling PHL-AMS and IIRC were trying to do a mileage upgrade from Y to J. That didn’t work out since J was full but an hour or two before the flight a PE waitlist appeared and they (and one or two other people IIRC) showed up on it, and they ended up in PE. I never asked what, if anything, they were charged for that upgrade… (neither had any AA status at the time IIRC).