Retirado do fórum do Forza
"Cars bought in Career mode are automatically upgraded to the top of the eligible car class for that series. You can back out to the main menu Buy Cars to buy them at stock PI (and for fewer credits) without the parts, and then buy the parts and upgrade as you see fit. There's a limit on each particular Career series..."
Basicamente se quiseres controlar os upgrades, tens que comprar o carro antes de entrar no modo Carreira.