When you return to the assembly room, you'll see bounding boxes representing the new terrains that will show up when you render. I advise rendering before doing anything else: this isn't a perfect solution (notice the abrupt cliff artifact circled in yellow in the finished quick render). You can either adjust the original terrain's edges or go back to the replicator editor and have it create real instances; this will make the replicated terrains their own objects. And possibly give you enormous files, but also objects you can individually edit.