The first generation of Apple Watch suffered from a few problems, including "ballooning batteries" that ended up displacing a few users' displays.
In response, Apple has quietly extended service coverage on first-gen device battery issues for an extra year. Now they have done it again in response to another issue, if their back cover separates from the rest of the device.
First-generation Apple Watch owners who encounter this problem will have a total of three years of coverage from the date of purchase. If your Apple Watch has this issue, you can schedule an appointment at the Genius Bar or an Apple Authorized Service Provider. Additionally, you can call Apple Support and mail the watch to an Apple Repair Center.
While Apple has circulated an internal memo on this topic, they haven't made the policy public. There are some reports that Apple Watch owners were required to pay for their repairs, despite the warranty extension. If you encounter this problem, escalating your request until you find someone who's aware of the policy should do the trick.