Kev,
If you have a new van with the fixed 30 mb bulkhead regulator then it's just a case of changing the pig-tail (the black hose that screws into the regulator) for the propane specific model.
If your van predates the fixed bulkhead regulator (probably pre-2004), then you'll need a new regulator to match the coupling on the propane bottle. You'll need to check the appliances for their recommended gas operating pressure and buy the appropriate value regulator.
Note:
a) Propane has around 88% of the heat content of butane, so you may (but probably not) notice that food takes a bit longer to cook.
b) Butane still works even if it is just below freezing outside - we were away at York last April when the temperature dropped to -3 C overnight - there was still enough pressure in the bottle to run the heater in the morning - but the internal temperature in the locker was probably warmer than -3 C.
c) But if there is continuous periods (day and night) of below zero weather, propane is best.
Why the difference? The boiling point of Butane is 0 C, below that temperature it's a liquid and like water just below it's boiling point not too much steam is released.
Propane has a boiling point of -40 C and it never gets that cold in the UK.
Conversely it's not a good idea to take Propane bottles to France when the temperature is +40 C
Robert