I think you can have ambi controls but it would be a bit of a work. The key thing to understand is your trigger "box" or "frame" combined with an unserialized "catch" or "auto sear" (folks use different language) and the firearm -- all together -- are the machine gun. The risk comes in separating any of the three elements and potentially creating two machine guns (one of which is "new," unregistered, and illegal). The option I believe you have is to take your trigger box to a suitable HK gunsmith and have it modified to accept ambi parts. You would need to be sure you reused your existing auto sear (or catch). In this scenario you have not separated the elements of the machine gun or created anything new. One potential issue is where your gun is engraved, if modifying your trigger box for ambi (cutting the selector hole bigger) damages the engraving I think you are out of luck. I am not a lawyer or SOT, and not everyone would agree with what I propose, but I would be comfortable with it.