Best option to do is to make a backup image of the OS before upgrading. Either Macrium Reflect or Paragon are good for this. If you have sometime and a spare drive, clone your Windows 10 to it and on that drive restore to Windows 7 and make a backup of it from there. Or you could probably backup Windows 10, and if ever you want to return to 7, change the date so that the day you want to revert to 7 it will still work.