How often are you supposed to change your car in Need For Speed Carbon?

It feels as though i have been using the same car for most of the game (an Aston Martin). Its winning me races but should I start upgrading a new car now- or use the Aston Martin for the rest of the game?How often are you supposed to change your car in Need For Speed Carbon?
Its completely up to u. When you start to lose with all possible upgrades then you change your car, or even change it when you get bored.. or fancy a challenge?How often are you supposed to change your car in Need For Speed Carbon?
use whatever car you want just make sure you buy all of the latest upgrades for it.
when its not possible to win races anymore

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