When I encounter the problem, this work-around has worked consistently. In Word 2003:
>Tools
>Options
>General tab
>check: "Confirm conversion at open."
Then, Word 2003 tries to open the problem file, stops, and asks me if I want to convert. After I assent, Word 2003 converts and opens the file. I can then edit as usual. When I save the file, the converter saves in the original format.
Then, to avoid the additional step that is usually not necessary, I undo the change:
>Tools
>Options
>General tab.
>uncheck "Confirm conversion at open."
A bit cumbersome, but it avoids the registry edit and has worked consistently over several years.