As described in the model assertion docs, the key "serial-authority": ["generic"] in a model assertion ensures that the public store generates a random serial for the device.
In my case, this ensures correct usage stats on the snap store dashboard, as the device is only counted when it has a serial. >90% of our users use a custom-built image based on a customized RPi gadget with an empty prepare-device hook. Is it possible to trigger this process manually from an already deployed device, or do users have to re-flash a newer image that contains the serial-authority key in the model assertion?