Chassis are specific to the yokes on the monitors, and certain chassis require certain tubes, but the majority will use the same tube. It's easier to answer this if you tell us which two monitors you have now.
Of greater concern, the frames are completely different, so you'll have to keep the Pac-cocktail frame and swap in the new tube and probably chassis, and the chassis will most likely require some mounting modification as it probably won't bolt on the same was as the other...