This is tucked into the announcement of the Android Wear watch face API, but it's an important one given that a good many folks out there with Android Wear smartwatches are using so-called "unofficial" or third-party watch faces.
Now that Google's provided an official API for getting watch faces to your wrist, existing watch face apps have until Feb. 1, 2015, to update.
Google also notes that watch faces built using the new official API won't be visible to users until they've received the Android 5.0 (API 21) update on their watches, so updated apps will either need to include the existing API Level 20 as a minimum check, or wait to update their apps once watches have the new software.
We recommend you update your apps on Google Play as soon as the Android Wear 5.0 API 21 OTA rollout is complete, which we'll announce on the Android Wear Developers Google+ community. It's important to wait until the OTA rollout is complete because a Watch Face requiring API 21 will not be visible on a watch running API 20. Once your user gets the OTA, then the watch face will become visible. If you want to immediately launch your updates during the OTA rollout, make sure you set minSdkVersion to 20 in your wearable app, otherwise the app will fail to install for pre-OTA users. Once the rollout is complete, please transition your existing watch faces to the new API by January 31, 2015, at which point we plan to remove support for watch faces that don't use the official API.
Source: Android developer blog