Both the AV and FSD file types come from the popular Final Draft screen writing software and it is likely only this program is capable of saving *.av documents to *.fds documents. No other 3rd party software seems to be capable of AV to FDS conversion.
Convert Final Draft AV document to Final Draft secure document file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert av to fds on Windows
A script writing software for professional writers
Convert from av file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.