Enable your watchOS apps to connect anywhere and anytime, even without a phone nearby, by updating for watchOS 4 and Apple Watch Series 3. Take advantage of increased performance, new background modes for navigation and audio recording, built-in altimeter capabilities, direct connections to accessories with Core Bluetooth, and more. In addition, the size limit of a watchOS app bundle has increased from 50 MB to 75 MB.
Apple also notes that starting April 1, 2018, updates to watchOS 1 apps will no longer be accepted. Updates must be native apps built with the watchOS 2 SDK or later. New watchOS apps should be built with the watchOS 4 SDK or later.