You gotta like that. "Most" of these are due to be fixed. Wonder what casued BA to switch over to this FMC when the Pegasus based FMC in the -400 works pretty good.
Spooky 2,
Already been answered by gas path:
The FMC's do not have the required amount of 'memory' for the aircraft route structure so they are being upgraded, together with FANS.
The other option would have been an absolute nightmare to manage namely an Eastern fleet and a Western fleet.
Thus, the change is happing because the 'Pegasus' is unable to store the current route structure. As gas path mentioned, separately grouping 52 -400s in two fleets would be problematic.
The -8 FMC has more memory and thus it is the perfect solution to the problem. And as Intruder has already said, the major firmware revision for the FMC is coming this week.