http://www.microsoft.com/office/editions/howtobuy/professional.mspx#EQE
See the part where it states:
Upgrade Eligibility
Determine whether your current version of Microsoft Office qualifies you
for the $329 US upgrade price. The following products qualify:
. Microsoft Office XP
Office XP Developer, Office XP Professional with FrontPage®, Office
XP Professional Special Edition, Office XP Professional with Publisher,
Office XP Professional, Office XP Small Business Edition, Office XP
Standard
. Microsoft Office 2000
Office 2000 Developer, Office 2000 Premium, Office 2000 Professional,
Office 2000 Small Business Edition, Office 2000 Standard
. Microsoft Office 97
Office 97 Professional, Office 97 Small Business Edition (version 2),
Office 97 Small Business Edition (version 1), Office 97 Standard
. Microsoft Works
Works 6.0, 7.0 or 8.0
Works Suite 2000, 2001, 2002, 2003, 2004, 2005, or 2006