Buyers of Office Home & Student 2010, for example, can choose either a perpetual-licensed copy of Office Home & Student 2013 or a one-year subscription to Office 365 Home Premium, the plan that lets consumers install and run Office 2013 on in a household.

Those who purchase the business-grade Office Professional 2010, on the other hand, will be able to pick between Office Professional 2013 plus a three-month subscription to Office Small Business Premium, or a one-year subscription to Office 365 Home Premium.

Since Microsoft will not be upgrading Office for Mac anytime soon, buyers of that suite will receive a one-year subscription to Office 365 Home Premium. All Office 365 plans let subscribers install and run five copies of Office on Windows PCs or Macs in any combination.

Some caveats apply: While Office 2010 will run on Windows XP, Vista, Windows 7 or even the upcoming Windows 8, Office 2013 works only on Windows 7 and Windows 8.

But the combination of the free deal and Microsoft's decision to give customers a way to save money: Purchase a copy of Home & Student 2010, for instance, which Amazon currently discounts to $99.99, and later receive a free copy of Home & Student 2013 that will list for $139.99.