If you use that model as a guide, it seems like it could be the same ship with completely new nacelles, a new "ring" around the outside of the saucer built on for additional room afterwards for the movie. The "neck" support was also built up and the torpedo area added on.
The phasers are a little harder to explain.
So if they did completely remove the engines and install a new engine system, that seems like the Phase II version, but then they "bulked" it up for TMP.
We don't really know how modular the basic structure is. It could be as simple as taking the edge of the saucer and adding more structural units and reinstalling the edge bulkhead. Same thing for the upright support, split it in the center and add framework to the new sections to make it wider, add more to the back to make it longer.