Yes, it's generally safe to back up game data or saves and transfer them to another operating system.
Yes, it's generally safe to back up game data or saves and transfer them to another operating system.
Imagine you have a Minecraft data file on Windows 8.1 and you wish to transfer it to your new Windows 7 system. Would there be any issues? The same applies to other games.
Sometimes it's okay to leave the game exactly as it is. I once had to reinstall Windows 7 on my SSD and ended up redownloading every single game since Steam and Origin managed to re-upload them all to my second drive. Another instance: My friends simply copied Kerbal Space Program onto a USB stick and gave it away. I thought, "Darn it won't work," but it actually worked surprisingly well. I can make any promises, though—good things can happen, and bad things can too.