I remember it being a big deal when folks started migrating to Office 2007 and some were still running 2003. Hence MS released the 07 conversion pack for Office 2003. But as far as Word/Excel I never saw a later version having trouble with an older format. MS-Access was most problematic in my experience with conversion.
Guessing it must have been MS-Access where you had problems. Access 97 format was a royal PITA as I recall...
And like Bill my experience with Open Office had a LOT more conversion problems across the board than just the database format. If you are invested in complex office documents (esp sharepoint) I wouldn't make the leap to Open Office.
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