Straight from Y to F can happen... I was on the VS17 last year, calling Virgin in the UK the day before I was due to fly with an excess baggage charge question I got a load of abuse for asking 'stupid questions' and the phone slammed down. Fortunately supervisor has been listening in, much apologising and promise of free excess baggage, and upgrade on the flight.
Turned up at LHR next day, and they realised I was on a Continental codeshare ticket. Continental doesn't sell 'J' on that flight they said, so the only possible upgrade was to F - upper class - which they proceeded to arrange :-)
Only time I've ever ended in F when flying on me own tab.
So far... :-)
R1