I guess a question remains if this is like an event horizon. Once you can see something in the distance that resembles the Mistlands, the seed is contaminated for the update and a new seed will be needed. On seed maps, Mistlands, Ashlands and Far North seem to be generated at the extreme 5 km edges of the map. So, one heuristic would be to remain within 5 km of the stone circle as we explore the other biomes. My seed had the first four Forsaken within that 5 km radius and I'll hold off on Yagluth until the update. I suspect that the update will require Yagluth to be engaged again in order to get any change to the "Yagluth Thing" anyway.