I don't think the Governing Body will ever announce this change from "the good news" to "the message of judgement." The judgement message requires the Great Tribulation to come. When the Great Tribulation is announced to have started, then it means that the end will come in just a few weeks or months. That would be the end of this cult because that end will not come.
Imagine this scenario:
Jehovah's Witnesses announce that the Great Tribulation have started. The Governing Body is gone as are all other anointed ones. Jehovah's Witnesses now await the end that would bring the Paradise, so they keep waiting for Armageddon. How long will they wait after the announcement of the Great Tribulation? What will happen if the end doesn't come in a few weeks? a few months? a few years? a few decades? Eventually, they will have to realise that the end is not coming.
Accordingly, if the Governing Body—or some other Jehovah's Witness-appointed team—announced the judgement message, all Witnesses would begin to anticipate the end... which will not come. That would be the end of this cult, I assume.
However, if it did happen—and I would love to see it, frankly—they would gain the same reputation as the Westboro Baptist church did. (By the way, how did that church even start? I mean... how the heck is this possible that anyone was ever convinced of this utter bullsh*t that this church sells?) The newspapers would probably be very busy calling the Jehovah's Witness organisation a hateful cult, etc. And then the Witnesses would be running around in their circles screaming, "Persecution! Persecution!" In the end, everyone would be disappointed that the end is nowhere to be seen, and so they would eventually leave. They'd be forced to join the real world. Without education. Without money. But that would probably take a year or two after the announcement of the judgement message. Maybe a bit more.
P.S. The young ones would probably leave the cult right after the announcement of the judgement message. They'd be too embarrassed, probably, to "judge people" in front of their colleagues from school. I know I would leave. Hell, I'd run for my life. Then I'd die of embarrassment.
P.P.S. The above gave me a bit to think about... We may see an increase in the suicidal rates of teenagers if the organisation ever announces the judgement message.