Minecraft wiki entry for Updating_old_terrain, associated text: It is much harder to find a smooth transition among breaking-change versions. A tool called mcmerge (Java 1.7 -> 1.8-1.12) can help by adding rivers around the transition, but you have to do some preparations on the old map before using it. If you don't want the preparation, bestofboth (Java 1.7 -> 1.8-1.12) simply looks for cliffs and fill them all in.