I suspect the Steam version will also "redownload the whole thing."
You may be right; I haven't updated it yet, as I don't have time to do beta testing, unfortunately. From what I've read, it looks as though it isn't necessary to delete the "old" version of the game, so I assumed it was just an update. Though for all I know, Steam could take care of the whole uninstall/reinstall process. I'll know for sure once the new version becomes mandatory.
With GOG, it's necessary to uninstall and reinstall the entire game, even if it's only an episode update, which is why I've been using the Steam version until the full game is out.
Oldman, that sounds like a good plan. Book 4 isn't too far away, probably sometime this month or early next month. So when Book 5 comes out, you will only have to worry about uninstalling and reinstalling once. Of course, if you don't want to do that at all, you may want to wait until the entire game is out.