SOLVED (Mods leave this up for people trying to figure out the same):
If anyone else is having this issue:
Download Paint.Net, open the old livery that has the license plate on it, then use the rectangle select tool and get the license plate (theres gonna be corners that you'll deal with in a second) then copy the plate you just selected, then open the new livery that you want on the car, then paste it (it should be pasted in the same spot so the license is recognized on the livery), then use the zoom tool and zoom in to the point where you see the pixels around the corners of the license plate, color it in based on your background, then replace the livery with the new one!