Just wanted to express my frustration with this “instant upgrade B fare”. I booked AMS - IAH - SFO and this was the return part of my trip. My Premium Economy upgrade to Polaris already cleared from AMS-IAH. IAH-SFO was booked as a B fare. When I go to check in for the flight, I am offered an upgrade for $399 which I decline. I called customer service and they didn’t know anything about the instant upgrade B fare. After talking with support, she tells me the flight isn’t available for the instant upgrade but can’t tell me how I know if a flight is eligible for an instant upgrade. I guess this is the PN thing I was missing but this whole thing sounds extremely misleading and there doesn’t seem to be anything instant about it.