I'm in the process of "manufacturing" an AR-15 piston SBR. Once it's all approved and whatnot, could I convert it to full-auto (or 3 round burst) if I wanted to?

I don't know what goes into doing that hardware-wise, so if you do, feel free to share. Mostly though I want to know the legal implications of doing so. Would I have to re-register it with the BATF or what?