sort of skirts around a similar situation.
My first thought is that the original file attribute has been set to read-only. Or that you are not recorded in its properties etc. as the original owner and again it is not letting you save to the original file name?
Also is the file in a format that your current version of Word natively uses? Although if you manage to open it via a converter - it will usually allow a save to same name but with a format natural to the version of Word in use; rather than that of the file source which you converted?
more recent versions of Word offer (default) .docx format; whereas earlier versions allowed .doc default only. Add in the free MS converter/utility and you may be able to save to this more recent (.doc) format too.
Also check this link/discussion too; referring to compatibility mode for Word...