ARS Technica warns Microsoft made a change to the perpetual retail license for Office 2013. While Office 2010 and before allowed you to transition the productivity suite to a new PC, the new Office 2013 suite will be locked to the computer you first install it on, forever.
If you buy a perpetual retail license for Office 2013, it will be locked to the computer you first install it on, forever. Buy a new PC and you won't be allowed to install your existing copy of Office on it, even if you wipe the disk of the old PC. You'll have to splurge for a new one.
This is a change in policy from Office 2010. Office 2010 permitted a single transition from one PC to a new one. It's not, however, an entirely new policy: OEM pre-installed versions of Office (and Windows) are similarly tied to their (OEM) hardware and can't migrate. Adam Turner at The Age first pressed Microsoft for clarification over what its "single PC" constraint actually meant, and noted the newly aligned OEM and retail licenses.