2 seconds on microsoft.com with the search term "office 2007 upgrade":
"Qualifying Products for Upgrade
Microsoft Works 6.0–10; Microsoft Works suite 2000–2006 or later; any 2000–2007 Microsoft Office program or suite; any Microsoft Office XP suite except Office XP Student and Teacher"
Office Home 2007 Retail License can go on up to 3 machines. Any Office OEM is the original machine only. A search for Microsoft MSLT will tell you the licensing terms for other versions of Office.