Theres some file you can edit and it will allow you to restore that backup to a old firmware, just Google it, and be careful make sure you make a copy of it incase it's get corrupted or anything.
It is sadly true, but if it is a newer restore you could take a snapshot of the older os and merge data from one restore to the other and then restore using the older os restore data. I have done this when I wanted to save my message database.
If you are trying with an iTunes backup rather than iCloud then you can. I am not sure which file to edit but you can easily open up a settings file in the back to update the iOS version. Google it!