Upgrades from Office 2007 and Free Upgrades
People who are using Office 2007 can upgrade to full versions of equivalent editions of Office 2010 at discounted costs. This means that if you are using a Home edition of Office 2007, you can get an upgrade to Home edition of Office 2010 only and not to the Office 2010 professional edition. Upgrading from Office 2007 will save you money as Microsoft the upgrade cost is far less than purchasing a new Office 2010 kit. However, if you are using Office 2007 Home edition and wish to have Office 2010 Professional edition, you will first need to convert your Home edition to Office 2007 Professional edition and then you can go for Office 2010 Professional edition.
You can also get a free upgrade of Office 2010 to full version, for some Office 2007 packages. The only condition is that you should have bought the Office 2007 between May 5, 2010 and Sept 30, 2010. Please note that you will get only an equivalent upgrade in this case too, but it will be free (only the costs of shipping will apply if you ask for a DVD).
If you are wondering why Microsoft is offering the free upgrades, there is a good reason behind it. Office 2003 and previous editions cannot be upgraded directly to Office 2010. You need to have Office 2007, Office 2010 beta, or Office 2010 Starter kit to upgrade to Office 2010 full version. The above time-period allows users to purchase Office 2007, understand the new Interface and then upgrade to Office 2010. Otherwise, you have to go for a direct purchase of Office 2010.
Important: The beta version of Office 2010 expires on October 31, 2010. If you upgrade Office 2010 to full version before the said date, it becomes easier for you as it would retain the customizations etc.