OS 5.0 includes a new interface navigation method and an interface with tweaks that take advantage of the higher-resolution screens, such as a new font and improved graphics. It can run older apps, although they’ll have to run in a compatibility mode.
The cutoff isn’t completely surprising given the scale of the update. Even so, it’s not a thrilling prospect if you own an earlier Fitbit smartwatch. Your device’s software might already be somewhat obsolete after just a year — while it’ll work as originally intended, you won’t get a steady stream of new features like you might from Apple, Google (while it waits to acquire Fitbit) or Samsung.