It will depend on the upgrade policy imposed by Microsoft for the next
version. I don't believe that the EULA for Office 2004 Student & Teacher
Edition states anything specific prohibiting its use as a qualifying product
for a discounted upgrade price (though I could be miskaken).
In the case of Office 2004, Office X Student & Teacher Edition is a
qualifying product for upgrade, but no one knows whether that will be the
case for the next version.