If you decide to drive a different car to the airport, then it's not an issue. Here you will find information regarding self and valet parking.
You can easily change the license plate in your confirmation email via manage my booking. Did you not have to submit a license plate at the time of booking? Then there is nothing to be changed and you can drive straight to the parking place.
With valet parking, it is important that you inform us of the change in vehicle, as the driver will be looking out for your car at the parking location. You can also call the parking provider to inform them of the change of car.