If the update breaks saved game files, then please allow people to decline the update and complete games in progress. One way to do this on Steam is to make the old version into a "beta" version of the game that players can optionally opt into, and then you can leave an update message explaining how to do so and letting players switch to the new version when they're ready. Having a surprise update kill all saved games in a single-player game is a rather nasty thing to do to your players.