The rules say I can only have one Steed (Vehicle, Ship) unless I have a card that says otherwise. Can I unequip one Steed to play another?

Steeds (Vehicles, Ships) cannot be unequipped at all and certainly not to play another one. If you can't use your Steed for some reason, you must get rid of it as an extra Big item. (If, somehow, you find yourself with a Steed [etc.] that you can't use but can't get rid of, it does count against your Big item total; you only get to ignore a Steed's Bigness when you're actually riding it.)