You can actually update geometry anytime you like. Export your scene as a single FBX > Import into the editor and set your level > Then every time you make a change export the scene again then right click > reimport the modified mesh only.
You can actually update geometry anytime you like. Export your scene as a single FBX > Import into the editor and set your level > Then every time you make a change export the scene again then right click > reimport the modified mesh only.