Windows preinstalled on hardware has never had downgrade rights. If you buy a computer with Windows version N preinstalled, you do not get downgrade rights to version N-1. Downgrade rights only exist if you buy Software Assurance.
In other words, if you buy 30 machines with Windows 7 preinstalled, you don't get to put Windows XP on them without SA -- it doesn't matter how big your business is (Unless, of course, you have non-OEM unused XP licenses lying around -- which is not what we're talking about).
The only "downgrade" rights you get with an OEM Preinstall is the ability to purchase an upgrade for that preinstallation and then fail to use the upgrade. That, however, would not be a downgrade, it would simply be someone wasting money on an upgrade license they have no intention of using.