At one time, you could upgrade your front regular seats to Next Generation seats, in any available color. The cost of the retrofit from Tesla was a little over $6000 for parts and labor (depending on your local labor rates). The labor effort is less than an hour of work.
Reference Tesla’s Service Bulletin SB-15-13-002.
The rear seat may also be retrofittable, but we don’t have any further details yet.
(Thanks to wk057 for the details.)
While not confirmed, these seats are out of production, so they are unlikely to be available. The latest seat design is not backward compatible, as they now have an electrically adjustable headrest.