On Wednesday, Tesla Motors dropped a bombshell—what it’s billing as full “Level 5” autonomous driving capability will be available on all of its new vehicles. But CEO Elon Musk has indicated on Twitter that there’s little chance upgrades to full autonomy will be offered to current Tesla owners.

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Those upgrade costs hinge on the distinction between software and hardware. Many new Tesla features, including recent upgrades to the Autopilot driver-assist feature in existing cars, are software updates and can be delivered wirelessly for near-zero marginal cost. But the full autonomy package represents a significant hardware upgrade from what’s currently standard on a Tesla.

For a start, the new cars will have eight cameras instead of one. Their onboard computers will be many times more powerful, to help them process all that additional information. And they’ll have upgraded sonar.

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That hardware, according to Musk, will be included when customers order a self-driving package with a new car, and will run $8,000. That price is shockingly low, considering that laser-based autonomous sensor packages have been estimated to cost tens or even hundreds of thousands of dollars by analysts.

But retrofitting all of it onto a current Tesla just wouldn’t be worth the cost and effort—and, considering how precisely calibrated things like cameras and sonar need to be to work properly, might not be possible at all.