Here is the 6' long first model built by ILM for 1984's Trek III. I HATE THOSE MEN IN THE PIC FOR BEING THERE (just kidding)! Behind the simple domed bridge structure in the middle of the saucer, is a singular and centered "Impulse Deflection Crystal". The bridge dome, the deflector crystal, and another dome-like structure behind the warp pylon connector are 3 of the more noticeable changes on the 2nd model.
The ILM model's coloring is also less severe, with a soft soldier blue decoration.
The second and smaller 2' model for Trek VI. I HATE HER (just joking)!
The newer bridge is smaller and more complex. There are two smaller deflector crystals replacing the one larger one. The plating, now blue AND yellow, surrounding the bridge area is more pronounced. The blue markings around the ship are a richer turquoise color in the newer model. While hard to see in these pics, that dome behind the warp pylon connector has been replaced with a curvy but more squared structure.
All of these pics come from another way cooler blog, BRUCE'S DOMAIN. If you like Star Trek and Starship model building, I recommend his blog. It's very thorough.