Tesla initiated and offered me an early lease-buy for my Model X if I bought/leased another Tesla vehicle. This was a little over a year ago. At that time, my Model X was 2.5 yrs into a 3 year lease. They gave me 30-days to accept the offer, which I did. However, it took the leasing company (US Bank) over a month to schedule a lease-end inspection. Tesla extended the offer by another 15 days. Other than that, it was a fairly painless process. I ended up leasing another Model X for 3 years.
I was over the mileage limit for 2.5 year mark but not for the 3 year mark. There was no extra mileage charge as my mileage was still within the original 3 year lease term.