It makes perfect sense that the improved/added features, along with bug fixes would be free. However..., when it comes to the DLC.., I'm not too sure as to whether it'll be paid or not.
The reason I'm on the fence is because 11 cars are NASCAR. I'm curious as to why they focused so much on NASCAR. Did PD's research show that NASCAR is being used in the game more than they had anticipated? I would only speculate that it is being used more for them to add 11 new cars specific to that style of racing.
I think people would be willing to pay as well. Especially considering how long users have been pushing the issue of DLC.