You can replace them with alternate tires, as long as they get back the original wheels. The tires are expected to be worn, but If they are shot you will likely be dinged for excessive wear. No idea how much they charge for that. I am guessing less than a set of new tires. But you may be able to buy a set of used tires for cheap. You are expected to replace the tires when necessary. The leasing company does a pre-return inspection, they look at the tires and measure the tread depth. You will find out then. My S lease was up with 45,000 miles, tires were replaced around 22,000 (flat tire requiring all four to be replaced, grrrr), but they were fine with mine.