When you change your car mode, it doesn't mean you lost all modification upgrades and have to re-purchase everything and customize it from scratch. When you change your car mode, all your performance and body upgrades weren't lost. They were just "uninstalled" All you need to do is install them back again However, visual modifications (vinyls, decals, paint, window tint, and wheel paint) were lost

eg. You have a grip mode Lancer Evo with a stage 4 engine upgrade. You change your evo into another mode (let's say speed mode. When you customize it, the car is still with stock upgrades. So choose custom upgrade. Choose engine upgrade. Scroll down and you will see a stage 4 engine upgrade and the price tag is not a number but a writing "owned". All you need to do is press x on that upgrade and it will be installed back free of charge.