I'm looking to install Gen 3 charger for my upcoming M3 purpose, I only need 8' cable now but will need 18' if I buy a second Tesla. Doesn't appear Tesla sells the cables separately, are they hard wired or can you swap them out?
They are attached but if you have the technical know-how and are comfortable, you can swap out the cable. I don't believe Tesla usually sells the cable on its own so you might have to buy one from another member online. There are also EVSE sites that will offer to perform this service for you.
Another option would be to just swap out the entire wall connector...or just order the 18' version to begin with. Yet another option would be to add a second wall connector if you ever add another car. Eventually, the gen 3 wall connectors will be able to load-balance similar to how the gen 2 wall connectors already can.
We have multiple gen 2 wall connectors which have 24' cables. This allows a lot of flexibility and redundancy as the cables are long enough to reach other cars in the garage. (WC1 can reach 3 cars. WC2 can reach 4 cars. WC3 can reach 3 cars.)
They are hard wired, replacing them is not worthwhile.
If you are preparing for a second Tesla, then install a 14-50 outlet where your second Tesla will be. If you are not outdoors and looking to save money, then don't bother with the wall connector at all. Just install the 14-50 and use the cable that comes with the car.