Hi - No problem. I personally have never changed back and forth, I did have the conversion parts and I did own a class 3 1919 in 30 06. My understanding is that you need the barrel - (obviously) and new booster. I'm not sure if the barrel sleeve is different though (length wise). You also need a new spacer where the belt passes through that allows the longer bullet access into the receiver. I think there is one more change you need in the trunion area, but I'm not 100% sure what that is.