As long as the type of vehicle is correct and your trip is on a domestic route, you will most probably not encounter any problem during boarding.
The standard procedure for changing a booking is to cancel the issued ticket and book a new one with the correct details.
However, if you are traveling internationally, you may not be allowed to board the ferry. You should thus make sure that all details are written correctly.
In case you are traveling with a rental car and don't have the license plate number yet, you can simply add RENT123.