Most ferry companies allow seat upgrades if there is availability.
You can simply upgrade your seat by using the online trip modification service through the "My booking" section. Kindly note, that in most cases, a price difference may apply for the upgrade. For last-minute upgrades, the crew onboard can assist you.
To change the size or the type of your vehicle, you can also use the online trip modification, given that it is allowed by the ferry company and there is enough garage availability.
For most Greek ferry companies, small and medium cars fall into the same category and, therefore, you don't have to proceed with any change. Furthermore, some ferry companies do not allow changes from motorcycle to car and vice versa.