If your outfit is relatively new and is fitted with a bulkhead mounted regulator then all that should need to be changed is the gas bottle to regulator pigtail. Older units tended to use gas bottle mounted regulators and if your van is fitted like this then a new regulator and hose will be required.
It is not advised that any such gas job be done as a DIY project unless you are suitably qualified so go to any reputable dealer to get the job done. Unless there are obscure warranty reasons, I don't see why such a change should have to be done by a Hymer service agent.