For now at least, the full upgrade target file will remain the ISO image.
If our Upgrader ever supports full upgrades, this might need to be adjusted.
Let's not spend more time on it now because that's unlikely to happen:

 - Our plans for changing our upgrade process are rather leaning towards
   dropping our own installer.
 - Even if we do implement full upgrade support in Tails Upgrader,
   chances are that it'll be easier to use an ISO than a USB image
   as input.