Different versions of Noteworthy Composer used different file types for storing songs, and .nwc files seems to have replaced the previously used .ntw files. This means that ntw to nwc conversion pretty much describes attempts to save older songs in a newer format.
We have not found any specifics, but it is likely that this should be possible, at least in some versions of the software. In any case, if you have issues with export or working with Noteworthy Composer files, it would be best to contact their support or some community website.
Updated: December 5, 2022