From what I recall there wasn't a button to ask if I wanted to save anything; but, that was a button where I could install it a different way if I wanted to. The basic install will save everything automatically.
For all the games that are installed via steam will do a one second or less update so it can change from win7 to win8. Other games such as old xp game that were installed via disc, 99.5% of the time it will work as it did before but, it might just reset the graphics so you'd have to go back in the game and change it. As for the .5% you'd just have to do a work around; just right-click the icon go into properties and search for "compatibility" and then change it to xp service pack 1.
All games all files everything will be...