Data point -- was on a Flex J fare (but really a United J fare that was pushed over to Emirates after they canceled EWR - DXB on my flight dates).

At check-in was told J was oversold and they offered $3500 for J ---> Gamechanger F

It was more than I wanted to pay but did give it serious thought since (a) this was corporate travel so the initial ticket cost was $0 and (b) 13.5 hours of GC F was incredibly attractive but ultimately declined because my main goal on this flight was to sleep.

Ended up being a good decision since I slept for 7.5 hours in the new 77W J