It's good. My only thing would be that the refit appears almost identical, except for the change in the impulse engine area detailing. Is that really enough to call it major? Maybe add something describing it to explain why a "major refit" is nearly invisible. Ideas would be that they merely upgraded the engineering stuff, the impulse engines being an outward manifestation, along with a new matter/antimatter reactor unit put in, but externally invisible since that's all in the guts. It'd be like upgrading the motherboard of a computer: invisible from the outside of the case, but a big difference in performance. My other recommendation would be to alter the refit's color scheme, looking more toward the Ambassador-era than the Excelsior. Say a change in the standards in hull plating or a new deflector shield grid or something led to it, it'll help alter the visual cues. I'd recommend a nacelle change as well, but that would require extensive modifications to the model, not just texturing, and thus would mean more work for you. If you do that though, you might take time to add a few small phaser strips (like off the Ent-D) to show a refit of the weapons, from pit phaser emitters to phaser strips. Otherwise, good model, interesting idea. Spell-check might be good before it goes up to the JoAT, but that's a minor nitpick at most.