+1 on orifice sizes being different. The ratio of gas to air is different for the different fuel gasses. Changing the orifice is how you get the mix right. The orifice is usually a small brass piece that either screws in or is brazed in somewhere near the air intake on the mixing gizmo.
As for the hose itself, I am not sure if it needs to be changed or not. It is my understanding that CNG operates at MUCH higher pressures than LPG, so if you were going from LPG to CNG, a hose change would almost certainly be needed. Going from CNG to LPG, unless there is some chemical incompatibility, I am not sure that you can't use the old hose. I don't know for sure that you can use the old hose, but I'm curious if you might be able to.