Microsoft typically offers its Office customers an upgrade guarantee program, allowing them to buy the current version of a product with a free or discounted upgrade to the next one being part of the package.
And when I went looking, sure enough, on the Office website you can “Buy Office now, get the next version free”
You know how we like FREE!
I already purchased Office 2010 and didn’t need to buy it. So how do we enroll for the next version free?!
Here is how to enroll in the program.
1. Buy and activate Office today
Purchase, install, and activate a qualifying Office product during the offer eligibility dates.*
2. Sign up for an email reminder
Provide your email address to be notified when you can redeem your offer.
3. Download the new Office, when available
After you receive your email notification, return to this website, www.office.com/offer, to download the new Office.
I ran Office 2013 in a virtual environment whilst testing Window 8.
ibannieb Tech Support 
January 29, 2013 at 6:40 am
[…] The next version of Office is now available for those of you who bought Office 2010 and are looking for your Upgrade Guarantee. […]