Terragen 4.6.43 introduced object replacing via right clicking the node's context menu in the Node Network view.
However, reloading or replacing an object's geometry doesn't update the existing internal node network of the object with the material/surface parameters of the replacing object.
This may seem counter intuitive, especially compared to other 3D applications performing this type of thing, but I think of it as a way of protecting any modifications made to an item's shader parameters for that particular item and project.
You are correct that loading the object into the project again (not replacing) will bring in the "additional" parts. You can open the new object's internal node network, copy the new parts, then paste them into the older node's internal node network; the one that only shows the original part.