While not strictly a downgrader, the Gotek SFR1M44-U100 with the "Flash Floppy" or "HxC" firmware is the modern king. It emulates a floppy drive but allows you to present disk images ( .img , .adf , .dsk ) at any density. It effectively downgrades your expectation from physical media to virtual media.

A simple toggle switch soldered to pin 2 (HD Density Select) of the floppy drive connector. By grounding this pin or providing 5V, you manually tell the drive "ignore the disk's hole; treat it as DD." This is the original "downgrader."

Many CNC machines bios-lock to a specific floppy controller signature. They will refuse to boot from a Gotek (virtual floppy) because the handshake timing is too perfect or slightly off. In these industrial cases, a true, clunky, mechanical FLP Downgrader using a real 34-pin drive is the only solution.