e.g. on tpe-hkg with 3 classes but not many people paying for F price.
they open it up for J passengers.
perhaps elite, and celebrities, etc.

2 years ago, before the new F was installed. i got seated in F section with a J ticket., in a supposedly 2 class flight. they seat singer Kelly Chan as well.

last year, i was in an F ticket, the cabin was filled so i think most of the rest are were upgraded. again, singer Faye Wong, her assistant and her manager took 4K,4G and 4D. i don't thk her assistant was in F ticket.

your case should usually arise due to change of equipment. the only case (regularly 2 class eq. sold as 2 class) i knew was A340 to DPS (my experience was last year, before the new F), me and another MPO silver were seated in row 1 & 2, upgraded from Y ticket. while a full fare J passenger seated at the back. we didn't ask for those seats.
but these were 'pre-determined' upgrades, i.e. they knew Y was oversold before check-in.