Before we get carried away let's be clear about what actually changed. Recall that Matt.24:45-47 is the scripture under consideration.
"Who really is the faithful and discreet slave whom his master appointed over his domestics, to give them their food at the proper time? Happy is that slave if his master on coming finds him doing so! Truly I say to you, he will appoint him over all his belongings."
Watchtower is now saying that this scripture refers to two separate appointments, one over "his domestics" and the other, "all his belongings".
In 1919, according to them, the slave was appointed over "his domestics". No change here.
In future (the Great Tribulation), according to them, the slave will be appointed "over all his belongings". That's the change.