It's no more difficult to change an automatic rifle that was "disabled" (changed to single and semi-automatic fire only) back to fully automatic, semi-automatic, and single than it is to change a "pristine" rifle (originally single and semi-automatic) of the same model to fully automatic, semi-automatic, and single so I'm not sure what distinction you're attempting to make. Primarily what they do is silver solder a filler plate in part the magazine well and replace the switch. That's how older M14 and M16 rifles that were sold by Uncle Sam were done. It was an easy matter to "undo" the conversion. A DIAS and Colt 4 position switch (the 4th position not mentioned above is 'safe') M16-AR15 Receiver Conversion is doable by someone with limited knowledge and/or experience. Pre-1986 DIAS's are readily available for about $200 US. Whether the gun is an older military weapon configured for semi-automatic fire after retirement to civilian use or a brand new one makes little difference.
John Thornton