In a notable recalibration of one of the auto industry’s most ambitious technological promises, Elon Musk acknowledged this week that millions of existing Tesla vehicles will not achieve fully unsupervised self-driving capability, even as the company’s financial performance is increasingly buoyed by advances in artificial intelligence and robotics. The admission marks a shift in tone for Mr. Musk, who for years has suggested that Tesla’s fleet — equipped with cameras, onboard computing and over-the-air software updates — would eventually be able to drive itself without human intervention. That vision, often framed as a cornerstone of Tesla’s long-term value, has been central to investor enthusiasm and customer expectations alike. But speaking during a recent earnings discussion, Mr. Musk said that hardware limitations in earlier vehicle generations would prevent a substantial portion of Tesla’s global fleet from reaching what the company calls “unsupervised Full Self-Driving,” or FSD — a system that would allow cars to operate autonomously without driver oversight. “Some of the earlier hardware configurations simply won’t be sufficient,” Mr. Musk said, referring to vehicles equipped with previous versions of Tesla’s onboard computer and sensor suite. “We will continue to improve assisted driving features, but full autonomy without supervision will require our latest hardware.” The acknowledgment could have wide-reaching implications. Tesla has sold millions of vehicles worldwide with varying generations of its driver-assistance technology, many of whose owners purchased or subscribed to the company’s FSD package with the expectation of continual improvement. While Tesla has long included disclaimers that its systems require active driver supervision, Mr. Musk’s earlier statements had often suggested that a software breakthrough could eventually unlock full autonomy across the fleet. Analysts said the clarification may temper some of the more optimistic assumptions about Tesla’s ability to deploy a vast network of autonomous vehicles using its existing customer