I dropped a question on the MS support web site to see if I have downgrade rights for the Mac 2008 Office and Student Edition I bought. We will see.
What I could find by googleing was that downgrade rights seem to be available mostly for volume licensing or pro or ultimate versions of MS software.
You can however use any install disk with any valid key, for that media, to do the install of the downgrade. But you must use a phone type activation and have a valid key for the version you are downgrading from.
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Glenn