For General Use/World Composition (UE4/5):
In Unreal Engine, go into Landscape Mode, select “Sculpt”, and find the Layer called “Heightmap”. Right click that, click “Export to file”. In World Machine, use the “File Input” Generator Device to import it and connect that device to a “Height Output” Output Device. Set up your Tiled Build options in World Machine preferences, execute a tiled build, and then import those files back into Unreal Engine as Landscapes. (For 1 to 1, you might need to play with settings on the “File Input” Generator Device. You can always tweak it back to where it was in Unreal if you keep your original Landscape in Unreal Engine, create another Landscape, and then play with Z Scale values on the new Landscape until it lines back up with the original).
For World Partitioning (UE5):
You can use the commandlet to convert your existing Landscape into a Tiled Landscape for the World Partitioning system.