Tesla Motors Inc. confirmed Wednesday it has a contract with South Korean battery maker LG Chem Ltd. for upgrades to the company’s first car, the Roadster, which has since been discontinued.

Earlier this year, Tesla began offering a $29,000 upgrade to original Roadster customers to replace the battery in the vehicle with a new pack and make other improvements. The new pack bumped up the range to around 400 miles, a 35% increase. Those upgrades are continuing today.

Tesla...