Letting some random guy install a manufacturer-discouraged hack onto the unit may indeed have serious safety consequences. Before you let him do it make sure he is a licensed-bonded contractor with a combustion analyzer (and knows how to use it), and is willing to sign up for the liability for any modifications made.
If he screws it up the risks of it blowing up or burning your house down are probably lower than those of improper combustion and major soot issues, along with extreme levels of carbon monoxide in the exhaust. Those things would show up in the combustion analysis.
Curiously, Navien sells kits for making those conversions on the NPE series, but explicitly states in the manuals for the NR series that it can't be field converted. See the warning on the bottom of p.13. Since it may require a software change in the control electronics or something, simply monkeying with the mixtures probably isn't a good idea.