No need at all to "purge" ; often the two gases are blended as in most European Auto fuel, BBQ gas and some gas cartridges.
What's needed to change depends on the vintage of your van.
If you have an "on cylinder" regulator it is a matter of changing that regulator. It's always wise to replace the hose every two years so change that to be safer.
If your modern and have a bulkhead mounted regulator its simply a change of the so-called "pigtail"; the hose that goes from the cylinder to regulator.[These pigtails are pre made assemblies, making them up is not a DIY job, but changing them ought to be.]
The appliances do not need altering provided you remain either as "on-cylinder" or "off-cylinder" as the van was originally built.
Don'nt worry about switching or purging the system. I've got one of each in the locker thro a multi regulator and can switch from one to the other with no probs at the turn of the tap. Mick
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Four weekends ago we were on a Caravan Club rally with the South Yorkshires at Torksey Lincs. Our neighbours had blue butane and their gas would not work as it had 'frozen', most others were seasoned cold weather campers and had red propane so ours was ok. We had used a padded aquaroll bag and lagged our pump pipes and our water flowed fine, others had to take their whale into the van to defrost, and had a devil of a job trying to empty frozen waste water out of the wastemasters!! We had used a bucket as it is easier to break ice on a bucket than inside a wastemaster!
I haven't seen the forecast - but was away last weekend at reighton sands (awful touring facilities) and will be away weekend after easter at CC site, so hopefully weather will pick up. as for the gas - chyanged over to red after couldn't cook tea!! the blue gas wasn't even gettin the kettle hot! - now no problems
think there will be a few of us struggling with butane this weekend as there is a lot waiting on calorlite coming out. I have 2 full 4.5 kilo butane bottles, hope it is okay