Thanks. As for the models, it's no different than various older and newer variants of Starfleet ships. TNG still had Constellation, Miranda, Oberth and Excelsior class ships in service (and a ton more, I'm sure). Same surely goes for other races. You can apply upgrades without throwing the space frame away. TNG, relying on physical models still, also had some issues with size. In some scenes it looked like the Bird of Prey was the same size nearly as a Vor'Cha or Galaxy class ship, which is absolutely wrong, not matter what sub-class they were. They are about the size of the Defiant-class... so yeah, subtle differences perhaps, but basically they are the same ship class.