I wonder if anyone can help with this situation because I can't get my head around it! We have a number of large word documents created in Office 2003 these are things such as news letters etc which are edited and re-saved each term / year. Having moved to 2007 last september we have been running into the problem that opening these 2003 documents working on them and saving them again as 2003 compatible will sometimes result in the document appearing corrupt in 2007 and sometimes they will open in 2003 but not always.
A temp solution has been to open the 2003 document in 2007 save it as docx before making any changes then continuing to work but this is a work around and not a fix. All the documents are relatively large and always contain multiple pages with pictures and tables etc. Sometimes when they corrupt and can't be salvaged with an old copy of word we have no choice but to use OpenOffice.org but this often changes fonts and messes up the formatting slightly so isn't ideal.
Was doing the same thing as you earlier this year when I found a lot of documents were going corrupt and the only way of opening them was OpenOffice. After playing around for a few weeks trying numerous different things, it seemed the problem always came back. We ended up saving as 2007 and as far as I know we no longer have a problem. So my suggested would be just convert them into 2007 and leave them that way if possible.
By the way, it only seemed to be with files which contained tables.
Thanks for the reply, nice to know other people have had the same problem there is nothing in the microsoft knowledge base and it's a difficult thing to google really! Rather than doing them all straight away I explained that when opening an old document to save it straight away before changes and that seems to work for the most part.