you have to replace the burner. NG burns much hotter, combusts differently, and thus uses a different size hole in each burner.
propane is a liquid, and the regulator lets it vent into gas by the time it reaches your burner, but NG is a gas.
its doable, but probably about as cheap as buying an NG burner setup vs. converting.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10