As mentioned above some things can not be undone. These things are Engine Tuning stage 1 - 3 weight reduction 1-3 and chassis reinforcement. Once done these stay with you car as long as you own it.
Turbos, exhaust, manifolds, air cleaners, suspension, tranny, diff and such can be reverted back to stock at any time.
So if you want the ability to return a car to stock do not use any of the engine tuning upgrades. do not lighten the car and do not re-inforce it. You can add the turbo or supercharger if available as well as any exhaust, manifold and air cleaner you like and of course drivetrain and suspension parts.
Race Mod also can not be undone.