Hi Speedster,
first try a different propane tank and regulator, your problem might be there. Does the tank/regulator work ok on your propane Silver B?
Secondly, the NG and Propane versions of the Genesis 1000-5500 series have different valves on the manifold for the different gas types.
It is not just a matter of changing the orifaces. The NG valves allow more gas to flow on the medium and low settings. If anything, the flames should be larger when you use a Propane orafice on a NG valve, as I understand it. When you use a NG orafice on a Propane valve, you run the risk of the flame cutting out on the lower settings. Not so bad in reverse.
Easier to get a NG manifold and swap.