Downgrade Rights

Please explain if all Windows PRO operating systems include degradation right or Dell do a special deal only for some models.
http://www.microsoft.com/licensing/about-licensing/briefs/downgrade-rights.aspx

http://www.dell.com/support/article/us/en/19/590895?c=us&s=dhs&cs=19&l=en

Explaining what I mean ... if the operating systems (Windows 8.1 pro) on all Dell Laptops has OEM licence or I must buy a new licence if the Hard disk is replaced by me for clear installation of Windows 7 pro and backup the OEM preinstallation HD for future use, when the programs that I used guaranteed to operate under 8.1 environment. What about Warranty of the product if I do the above?