RealFish has it right - IF booked numbers in Club class justify it, then the First seats may be opened up (usually to the higher 'levels' of frequent flyer first), but with Club levels of service. Often referred to as 'Flub class'! Thought I had got it for Mrs T and I a while back, changed the seats on the booking, but got knocked back - luckily to our original seats - a couple of days later
The flight went with a fairly full Club cabin and empty First.