Can I restore an older iOS backup onto a device running a newer version?

This question was asked a while back and I think what the guy was asking was not answered right so I hope he liked his update. The guy in question as well as myself I believe is asking if you do a backup is your operating system saved so that if you don't like a new version, you can go back to your older iOS by using the older backup. People were answering yes but I think they mislead the guy. I think they are saying you can use the older backup, but the operating system stays with the newer version. It was made to sound like you can use backup to go back to how your phone was setup at the time of the backup, with both operating system and data. I would like to be assured I can do that, but I am pretty sure you can't. So, is there any way to go back to a past iOS version?