You'll need a 3rd part and money.
Say you've just won an 4 star road class engine for your current car (also 4 star) which has a 3 star engine. You install the new engine and then are ask would you like to keep the old part, it'll cost you $xxx or xxx Gold. Pay to keep it and then that 3 engine is kept so you can move it down to a lower ROAD car that has a 3 stars. Repeat if you've got another car below that.
