I have recently been trying to swap a Calor 7kg (blue) butane bottle for a Calor 6kg (orange) propane bottle. In theory Calor will allow swaps as posted above, but currently they won't. There is a national shortage of gas bottles, especially of the smaller sizes. I have spoken to both Calor and a local Calor agent and they will currently only swap like for like: same size and same gas type. Not only that, Calor will not enter into any new contracts; at least for the smaller sizes.
I don't know exactly what has caused this current bottle shortage, but I believe it is mainly down to the past two year's lock downs. People have gone to 'garden living' and are busy polluting the atmosphere and wasting energy like mad trying to heat the outdoors with their patio heaters. They hoard the bottles so there are not enough empties being returned to supply the exchange market, let alone the new customer market.
For information, the left hand thread on the propane bottles is called a POL thread. There seem to be two versions of this that I know about, one in the UK and one in Australia. The Australian POL male fitting has an O ring near the tip and is a slack fit in a UK female fitting. The UK POL male fitting has no O ring, and will not fit into an Australian female fitting. Perhaps someone can enlighten us more about POL threads?
David