While I may not have this entirely correct (I am a controls engineer, not a combustion engineer), my recollection is that naphtha is a difficult liquid fuel - it has low lubricity (compared to distillate or kerosene) and is corrosive. I believe that it was preferable to leave the better fuel in the system while running on gas or when the unit is shutdown. Then, when either changing from gas to liquid or starting up, you begin with the kerosene or distillate, get a stable flame established on the liquid system and then transfer to naphtha.