You can only have two (2) vehicles listed on your Park Solitude permit to switch between.
If you have added two, and you get a new vehicle, please contact HONK support (support@honkmobile.com) and request that we remove one of the vehicles to add the new one.
Removing the plate from your account, does not remove it from the permit unfortunately. Reach out to HONK Support to get an additional plate change sorted.