Thirdson,
I do not think it is new light - I think it is more of a slip as to what the actual situation is. The official teaching is that the anointed class, as a whole, is the slave. The individual anointed ones are the "domestics" of the Lord's house, while as a composite whole they constitute the "slave" who is appointed over the house. But in reality, the average anointed one has no say in the governing of the organization. This is actually carried out by the Governing Body. So for all practical purposes the Governing Body is the slave, and this is the reason for the "slip" in your quotation.
It's important to realize the psychological impact of having such a "slave." It's like the magisterium (or teaching authority) in the Roman Catholic Church. This authority is vested in the pope and the bishops, not the average Catholic.
When Russell was considered to be the "slave" (or "faithful and wise servant" - KJV), it was evident that the average Bible Student (or JW) was not the slave. When Rutherford eventually changed the understanding of who constitutes the slave, I believe he said that "the Society" is the slave. (If I'm wrong, someone please correct me.) But this was not satisfactory scripturally, because the Society is a legal corporation that cannot be found in the Bible. So the current understanding that all the anointed constitute the slave was adopted when it became apparent that the "other sheep" would far outnumber the anointed, and hence the average JW could not claim to be of the slave class. This tactic of keeping the slave distinct from the rank and file has the effect of preventing them from doing their own thinking. If they were all the slave, they would start coming up with their own explanations of scripture, and Big Brother certainly wouldn't like that!
So at this point, I don't think there has been a change of understanding. Someone on the writing staff just slipped up and identified who the slave really is!
Justin