Always read the ReadMe file when available, but if you take all the files from the new downloaded folder and move them to the old games folder and overwrite where necessary, it will leave your old saves alone. Since the new game has no already created save files, there is nothing to overwrite your old save files with, if that makes sense. To be safe, you can copy the whole old folder so that if you mess it up you still have the original to try again with.