what's going to happen if i..... ?

Jimmy, do you mean you want to replace just the faceplate from the head unit of your yellow car with the faceplate from the red car? If you plan on just swapping the faceplates, I don't think it should be a problem, nor should you need to enter the radio's security code. Only if power is cut from the head unit will you have to enter the code.

If you plan on swapping the entire head units, you will need the codes once you plug in the power. You'll also need the "key's" to remove/pull out the radio from the car.

Perhaps see if any others chime in with their thoughts, but I'm pretty certain I'm correct.