Unfortunately, we have not found any relevant information about flx to igs conversion, beyond the fact it probably has something to do with with export of old Autodesk FLIC animations or at least attempts to. Some software still claim to support FLX format, or did in older versions, so these are probably your best shot. But of course if you are looking for something specific, you might need to consult some specialist forum or perhaps Autodesk support.
Updated: July 21, 2023