Both .fds and .av files come from Final Draft script writing software, with the key difference here being that .fds files are saved in 'secured' format and thus cannot be edited nor saved to different format. This ultimately means that no fds to av conversion is doable. If you want to resave the document, you have to obtain the source file from which the .fds file was saved in first place.
Updated: March 6, 2023