Three 777's were purchased in a hi-density config for TPA/ MCO/ ANU/ TAB/ GND/ KIN, if I recall correctly they were V-IIO, IIP & IIR. They were "remodelled" after lots of complaints due to the 10 abrest seating in Y.... But to answer your question, now they are in a standrd 3-class config, there is more scope for them to be swopped around in the (GATWICK) fleet. It CERTAINLY happens, but I have never seen FIRST tickets; The cabin is sometimes NOT USED or if a busy flight, then Club pax are moved up, but are given Club catering....
There is a trip report about this happening on flyertalk if you can dig it out....
But then again, there is all sorts of RUMOURS about the 777's going to LHR, 767's coming (back) to Gatwick, and of course the Thomas Cook RUMOUR as well.....
I think you would find it hard to establish when one of these substitutions is going to occur!!