Tesla jumped after rolling out a self-driving update for owners of its older cars, which is good news for the stock and for drivers who felt left behind.

• Tesla stock popped 8% in a day, helped by a software win and a friendlier overall market.
• Older "Hardware 3" cars, stuck without updates for 15 months, finally got a slimmed-down version of Tesla's latest self-driving software.
• The fix matters because Tesla had hinted these older cars might never run full self-driving, sparking a class action lawsuit from angry owners.
• The update was squeezed down from the newer software using a copy-the-teacher trick, so it works on weaker, lower-memory hardware — not perfect, but a big improvement.
• It also saves Tesla money by avoiding costly retrofits or new "micro-factories" to upgrade old cars.

Outlook: Tesla deliveries are due this week, and growth in its core Model 3 and Y is expected to slow sharply, leaving the high valuation resting on future bets like Cybercab and Optimus.