Yesterday I checked in on the phone app to see if there were first class seats available for upgrade purchase exactly 24 hours before the flight. It offered MCS for $179 but F wasn't available. Just to make sure I called in the gold line to "upgrade to F." The rep apologized and said all the F seats were taken, but that since I'm Gold, I get MCS for free, and asked me which seat I wanted (without any suggestions from me - the rep just said it). I was ready to shell out the $179 so really lucked out by calling in to see.