No you dont have to buy the tires, once you own them you own them, which is what makes everything else so puzzling. I was talking about aero parts, those cannot be switched out at will without buying the part again.
If you spend 3,000cr for a spoiler and install it, to take it off you have to go back to GT Auto mode, pay 100cr to switch back to the standard one. But what's real funny is if you want to put that spoiler back on THAT YOU ALREADY BOUGHT, you need to buy it again. I cant help but laugh every time I think about that. This game is a tuners nightmare, because its fun for us to swap parts out, do some test runs, check the lap times, swap some more parts, do it again, but this game actually seems to not want you to do that by charging you FULL PRICE for these parts every time you want to put them back on. Basically if you love tuning and testing cars, this is probably the last sim in the world you should consider buying.
Im convinced Kaz is just out of his mind with some of the worst ideas ive ever seen in any racing sim, ever.