What I don't understand is why not all cars have tessellation. I have a feeling it's because of the PS3's power, but if that's the case then why have the tessellation at all? Also how come almost all of the cars that are new to the GT franchise have tessellation but some of the older models don't?
It seems likely that the the tessellation is the result of a new workflow for content creation. Or, rather, the new workflow is the result of the decision to use progressive meshes and tessellation in the car models moving forward (PS4). Getting those rendering processes to work on PS3 was a no brainer if they could pull it off; they have the assets, why not show them off?
PD have a different modeling team structure in the credits for GT6, as compared with GT5, and added a few support roles to that as well - including
tool programming and modeling technical support (for a team experienced in making models for the PS3?).
It should be a case of going over the meshes made for the earlier games and defining topological regions, then setting each region (e.g. a closed loop of edges, a "spine" or "bridge" between such loops etc.) to represent a certain vector primitive (like a straight line or a parabola; think CAD data), and storing that information in a new file-type.
Within that new file-type, the existing polygon mesh data can be stored in such a way that it can be pulled out face-by-face and gradually add detail up to the highest LoD (before then subdividing certain regions according to the pre-defined vector primitives). That re-sorting of storage order can be largely automated.
The topology and vector primitive work is likely to take some effort for all the other Premium cars, but nothing like modeling them from scratch (but still more than doing that "extra" work whilst you model the car in the first place). Interiors will be particularly burdensome...