The popularity of cloud-based application is getting bigger, and Microsoft is trying hard to promote its own Office 365 application. To do that, Microsoft changed the Office 2013 policy which wasn't accepted well. Under the original license terms, you couldn't install a retail copy of Office on more than one computer as the license is tied to that one machine. This means that if you install a new operating system or maybe lost a notebook, you will have problems with Office 2013 reactivation.
However, it seems that Microsoft still listens to customers. After public disapproval, Microsoft updated the Office 2013 terms and condition to allow transfers. Microsoft now allows one transfer every 90 days unless the transfers are due to hardware failures. The change is effective immediately, but if the activation fails, you will have to call customer support.