In some ways it was an odd thing to change.
In theory all anointed were part of the faithful slave. But in practice only the GB had the authority of the faithful slave. Everyone kind of realised that anyway.
But to actually change the doctrine in order to make that explicit. It just seems heavy handed and so unnecessary. Plus it raises really awkward questions about who was the slave earlier in JW history and whether you can have a slave that doesn't realise it's a slave and others who think they're a slave who are not. It's just a mess.