Open 2008 files in 2007?

We have 14 2007 licenses, and 12 upgrades to 2008, with no money to buy two additional upgrades. Will the two people still using 2007 be able to edit files from 2008? Maybe it's a case of, they can open the file, but any 2008 features won't work? Or will 2007 not even open the 2008 file at all?Thanks!Sandra

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