From the Devs

“I really hope people will, instead of just continue playing with their own worlds, actually create a new world and start from the beginning.”

"This is partly due to much of the new content only applying to unexplored areas – for example, Svensson shares “we added some new locations to the Plains biome, and [if] you have already explored parts of the Plains biome, those areas will not be changed. You will have to go to unexplored areas to find the new things”.