First the neutrino, now the kilogram, is nothing safe!!!
However:
The idea that they can know it changed by 50 microgrammes , but they are not sure whether it is +50 or -50, strikes me as very odd. Or maybe I read it wrong?
There are a bunch of copies of the standard distributed to different countries around the world, and periodically they compare the primary one with them. They're all fluctuating over time, but since there's nothing else to compare to, all they can actually tell is that the prototype has dropped relative to the rest. If they've all lost mass, the prototype has lost more; if they've all gained mass, the prototype has gained less. It's theoretically possible that the prototype has actually stayed stable, and only the copies have changed, but this would be quite a coincidence.