It's your phone...
... but the software on it is only licensed, not sold, so you're restricted by the software license. At lease, that's how I understand it all to work.
iPhone backups only work with the OS version they were created with, plus newer versions can restore from them. They don't contain the apps, just the data. It's possible to pull the data out from a backup and restore it into an older version of the OS if you're jailbroken and bored, but typically it's more effort than it's worth just to get your high score in a game. For real apps they typically sync with a desktop or cloud so your data is easy to get at.
The article implies that only going upwards in terms of versions is a bad thing. I disagree... better that they spend their resource fixing the bug quickly rather than having everyone "dipping their toes in" and the potential of failed reverts. Upgrades typically change data formats (for internal data) so a revert would have to roll that back. You might have upgraded an app that only supports the new OS, etc.
If it were easy everyone would be doing it.
Lets turn to a Microsoft OS software upgrade. You upgrade your version of Windows from Vista to Windows 7 in-place. Is there a way to back out? A simple way?
Same for OSX. If I'm on Leopard and in-place upgrade to Snow Leopard, can I roll back easily?
i.e. upgrades typically are difficult to revert from.