http://www.microsoft.com/oem/en/licensing/sblicensing/pages/downgrade_rights.aspx#fbid=sLMLnUlmfw8
Understanding downgrade rights
Downgrade rights offer a practical solution. The customer can
Purchase a PC with Windows 8 Pro preinstalled.
Downgrade—at no additional cost—to Windows 7 Professional.
Move to Windows 8 Pro at the appropriate time. The system is licensed and ready for conversion.
http://www.howtogeek.com/133168/how-to-downgrade-windows-8-pro-to-windows-7/
How to Downgrade Windows 8 Pro to Windows 7
Downgrade rights are only available on computers that come with Windows 8 Pro. Upgrade copies of Windows 8 Pro don’t include downgrade rights, so you can’t purchase the Windows 8 Pro Pack to get downgrade rights.
You can only downgrade to Windows 7 Professional or Windows Vista Business, not Windows XP. (For the purpose of this article, we’ll assume you probably want to downgrade to Windows 7.)
After downgrading, you can reinstall Windows 8 at any time.