Kerry, sorry to dig up this old thread but I've just had an occasion to find a way to update a single object. My Google search led me here. After a bit of work to change your last solution to work in Acad2014 or 2015, it runs without error but also doesn't update the entity.
Background for my needs....I've been coding for Civil3d 2016 lately. I found that I have some tools to edit Aecc objects, but they were not updating until a REGEN was issued. I didn't want to add a full REGEN due to these drawings usually being fairly large and a regen is often a bit slow. In 2016 that isn't a problem, because I found the Acad.Utils.RegenEntity() method which updates the objects just as I needed. But then I opened the projects for C3D2014 & 2015 (we don't support any of the older versions with new code) and was greeted with the old RegenEntity() not found error. Hit Google looking for something I could do, found this thread and it's sister over on the Autodesk .NET forum.
So, back to my current issue with this code. It seems like it should do what I need, but it is not updating the entities like the 2016 RegenEntity() method does. I also tried Tony's mention of changing the entity's layer to the same layer, which also doesn't update the object. Did this work to update objects for you?