The OEM licencing agreement of Windows 8 is quite different from Windows 7.
With Windows 7 OEM licensing the licence allowed you to install it on one pc, at which point it would lock to the motherboard and could not be transfered.
Windows 8 has changed this restriction. Windows 8 OEM can be installed on one PC at a time but unlike Windows 7 it can be transfered to another machine you own or even transfered to another owner providing the original installation is removed/deactivated. In the case of owner transfer it must be transfered in its entirety, that is to say the media, COA and original proof of purchase must be transfered.
The only restriction regarding Windows 8 OEM is on machines that have the OS installed by the device manufacturer, these cannot be transfered.
Further details of the Personal Use License can be found here http://personaluselicense.windows.com/en-US/default.aspx
I don't think that it applies to the upgrade version as that license requires that a qualifying OS be present on the machine at the time of install, but cant be 100% sure.