Almost always when you download an update, it'll be saved on your computer as its own separate game. So you'll still have your old copy of the game with all of your saves, and you'll have a new copy. Sometimes the new copy of the game automatically sees your previous save files, but if not, you can just go to your old copy of the game, and copy and paste the SAVES folder into the folder containing the new game. Very simple. I've never had any saves be deleted unless I've deleted them myself on purpose.