Perhaps you should buy and try befor replying. Losing all the pats for a build only happens in Forza Motorsport 7 and no other Forza game Horizon 3 was the last Forza game to have a AH and you did not lose any upgrade parts with cars you bought from the AH. Only the Tune settings if you wanted to change it. And this only happens with Cars you buy from the AH in Forza 7 if you Down load a locked tune on to a car and then change it you do not lose any build parts.