Final Draft professional script writing software used several "main" document formats throughout it's existence, including .fd, .fdr, .fcf and finally .fdx files.
So fd to fdx conversion is probably just a matter of re-saving old Final Draft document to modern format, which is probably doable in any version of Final Draft that already features FDX format, Final Draft 8 or later. But it might be possible that very old formats are no longer supported in latest version of the software, so if you struggle with such import or need help, it would be best to contact Final Draft support.
Updated: January 20, 2023