Well, even if you were to keep the appdata for saves (which I also approve of, btw!) include a saved game with a text file explaining what it is, why, and where to stick put it. This could be in the first release with the new engine (Unity or whatever if that changes...), kind of an apology for the change of game engine, but won't come with future editions (if they're regular players it's a reward for sticking with it - if they're not then they don't need it!). Same solution if there was even an update that had incompatible saved games!