See here, mate:
There have also been changes in the “Aero and Appearance” section of the Upgrade Shop. Most cars with distinct, standalone front and rear bumpers now offer an option to remove the bumpers. For a more period-correct look, specific models receive vintage motorsport-inspired front and rear aerodynamics, and add paintable headlight covers for a race-ready look. Some muscle cars now offer aftermarket drag racing style hood options in both Pro Stock and classic cowl-induction configurations.
You’ll also have the ability remove the factory rear wing or spoiler from some cars, and for the ultimate upgrade, some cars allow you to purchase OEM appearance packages available only from the factory, completely transforming the look of your car. While not every car has each of these options, the new options and features are going to open up a new world of customization options for racers, painters, and photographers to experiment with.
http://forzamotorsport.net/en-us/underthehood3/