I have just transferred files from a G4 Power PC to a new Mac mini and installed a new Office2008 for Mac (H&S ed). I can open some old files OK but not others. They open but do not display text. The blocks are there and red and green underlines show in places, but that is all. Office and OSX software updates are fully up to date at time of writing.
Comparing "get info" for two docs of similar size, one that opens OK and one that doesn't, shows no perceptible difference. Both are MS Word 97 - 2004.
The problem in CyberTaz's response lies in the "should open normally". They don't, although 'compatibility mode' is displayed. So I too would like to "convert" them so they do open properly!