Carpool reservations are for vehicles with 4+ passengers and qualify for a discounted parking rate. Oversized vehicles do not qualify for discounted carpool reservations.
If you arrive onsite and have less than 4 passengers you will be asked to pay onsite for the regular reservation rates.
You can't downgrade (or upgrade) your reservation once its been purchased. However, you can cancel and re-book for the appropriate type of purchase desired.