Is the Public Test save file compatible with the new Early Access version after the patch released?
Yes! When the Public Test version moves to Early Access, your Public Test save file(s) will continue to work on the new stable version of the game.

So when Mistlands drops to the early access, I can just revert to the early access, load the game and the save from the Public Test will be there? Or do I need to move some files or restore some backups?