VCC2 I don't think that is why.
It is something BA have always said was going to happen and as the Las Vegas goes to a 747 in June, they have decided to do it ready for that and I am sure they will add in other routes too.
I doubt, very much, that they will roster VCCs on Mixed Fleet 747 flights because many of the M/F crew will themselves be new to the aircraft and those that have experience of the 747 will, I would have thought, be busy helping their own Mixed Fleet colleagues get to grips with a new aircraft type.
These are of course my own personal views and I don't speak on behalf of my employer BA