If you don't change any of the shading groups (or their names) you should be able to update or replace the mesh through the obj reader. If the filename doesn't change, just reopening the scene should update it.
If you make any changes to the shading groups (using Maya terminology here, they may be called something else in Lightwave), then you will run into issues. TG stores all the shading info in the tgd file and won't update it when you replace the mesh. So you can get mismatches and errors.
I've learned through experience to work defensively when it comes to imported objects.
First, I always check the mtl file exported by Maya and prune any unused shading groups. (Maya exports every shader in the scene, whether its used in the mesh selected for export or not.) This avoids having your object's internal network cluttered up with unused object part shaders.
Then, as I'm building the object's shading network I try to arrange the nodes so they can be easily selected *without* selecting the object part shaders. When the network is finished I then select all the nodes (minus the part shaders) and export them as a clip. This way, if I ever have to rebuild the network for a new version of the mesh, I can insert the clip in the internal network and just reconnect the object part shaders.
Sometimes you just have to rebuild the network, and this really speeds things up. Plus the clip file is a nice way to back up those networks. (As you probably know, they can represent hours of work.)
Finally, if you don't already do this, you might want to consider using tgo files instead of obj files in your final scene. tgo files include the shading networks and don't use mtl files.
So my workflow goes like this: Every new mesh is imported into an empty Terragen scene, then shaded. When the network is done the scene is saved in the same folder with the mesh, and the object is then saved out as a tgo file. The tgo is then placed in the final scene.
It may seem like I'm adding an extra step (and I guess I am), but shading the mesh in an otherwise empty scene is much faster then working in the final scene (which just keeps growing in size). It makes it easy to try different lighting angles, etc. And tgo files are pretty bulletproof. The downside, of course, is that if you do make changes to the mesh you have to make a new obj *and* tgo file. (Which is why using shortcuts like clip files can really help.)
I'm curious to hear how other (more experienced) hands deal with imported objects.
