Tesla CEO Elon Musk said the company will “most likely” begin upgrading older electric vehicles with its new custom chip later this year — a lofty task that will involve retrofitting hundreds of thousands of Model S, X and 3s.

Musk tweeted Sunday night that the upgrades will begin most likely at the end of the fourth quarter.

Musk didn’t provide other details. He has previously said the upgrade would be free for owners who purchased the full self-driving feature, a software package that costs $6,000.

Tesla offers two different advanced driver assistance packages to customers: Autopilot and Full Self-Driving, or FSD. Autopilot is ADAS that offers a combination of adaptive cruise control and lane steering and is now a standard feature on new cars. FSD includes Summon as well as Navigate on Autopilot, an active guidance system that navigates a car from a highway on-ramp to off-ramp, including interchanges and making lane changes.

While Tesla charges for the FSD software package, the vehicles are not fully autonomous. Musk has promised that the advanced driver assistance capabilities on Tesla vehicles will continue to improve until eventually reaching that full automation high-water mark.

Anyone who purchased full self-driving will get FSD computer upgrade for free. This is the only change between Autopilot HW2.5 & HW3. Going forward “HW3” will just be called FSD Computer, which is accurate. No change to vehicle sensors or wire harness needed. This is v important. https://t.co/lICMpT7xnX — Elon Musk (@elonmusk) March 29, 2019

The custom chip unveiled in April has been couched as a necessary hardware upgrade to reach that goal. Since March, new Model X and S vehicles have come equipped with the chip. The Model 3 followed a month later.

The custom chip was a milestone for the company. However, it still faces the considerable challenge of upgrading thousands of so-called “Hardware 2” vehicles, not to mention the continuous development of the software.

Tesla started producing electric vehicles with a more robust suite of sensors, radar and cameras — called Hardware 2 — in October 2016 under the premise and the promise that it had the hardware needed to eventually drive autonomously without human intervention. At that time, the company also began selling the upgraded full self-driving package that Musk said would eventually reach that ambitious target.