This is a tiny bug-fix; the system will now recognize end of game saves generated both from the original game (which had an extension of .sav3) and the New Game+ version (which has an extension of .ng3). This means you can finish a normal game in Book 3 and use that save in Book 1 without any manual file renaming shenanigans.
Sorry it took a bit of time to finish, been AFK for a while. Enjoy :)