An already-paired key card is required to pair a new phone, though, so you probably won't want to throw them away.

What are the cards like? Just simple passive RFID chips?

They're not passive. If they were, you'd see a lot of articles about Teslas being stolen after someone walking by used an RFID reader. They contain a private key to create a challenge-response system with the car that essentially makes them impossible to clone.

I've seen articles of people using Java cards to create a Model 3 key card that works, but it wasn't possible to clone an existing card. They created a card, then had to use an existing authorized card (or phone) to unlock and turn on the car, then use the car's tablet screen to add the created card to the list of authorized cards.

I meant passive in that they didn't need a battery

Oh. Yeah in that sense, they are passive.

The cards are Tesla branded passive RFID Java Cards, you can confirm this with the NXP RFID reader app on Android. They are powered by the RFID reader.

Technically, as of this moment, you cannot buy more online...