Why...why didn't you do that in the first place, Polyphony Digital? Did you run out of time? No, that can't be it.
Before I start ranting, an upgrade of Gran Turismo 5's standard cars to premium models is not yet confirmed, but Polyphony Digital's Kasunori Yamauchi has mentioned the idea. In a Famitsu interview, Yamauchi said that there are plans to upgrade the standard cars to the high level of detail that the game's premium models feature.
As CVG says, we're not sure if this means all cars or just a few. I hope they redo all the cars. Some of them look assy in comparison. I feel like redoing all the cars would be a lot of wasted energy that they could have spent on making GT6, though. While I'd like all the cars to look their best, it's not like any of them look terrible, and the game would not be any more fun with these touch-ups.