Unless the update is fixing critical bugs, the odds that people already playing will update is low.. esp if they see a warning that their save will be cleared.
Best bet is to hook new players who haven't sunk in countless hours.
Another method is if possible add teleport locations for players who already played the game to warp to where they left off.. ofc if your game allows it and doesn't break the game aka via skipping important areas that have to be played to continue etc.