See here:-
http://www.microsoft.com/office/editions/howtobuy/standard.mspx#XSLTsection127121120120
Upgrade Eligibility
Determine whether your current version of Microsoft Office qualifies you for the
$239 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 Suite 20061, Works Suite 2004, Works Suite 2003, Works Suite 2002, Works
Suite 2001, Works 2000, Works 8.0, Works 7.0, Works 6.0
1 Planned versions of Microsoft Works (up to Works 8.0) and Microsoft Works
Suite (up to Suite 2006) will qualify customers for the upgrade versions of
Office 2003 Editions.
Notes
..Microsoft Office 95 doesn't qualify for the version upgrade price of Office
2003 Editions.