It is possible, and safe to transfer liquid propane from one tank to another, but I doubt everyone is aware of the potential hazards.
Tipping a tank upside down introduces the possibility of it falling and breaking the valve or the hose and this will release liquid propane which flashes into large quantities or propane vapour which is heavier than air and can travel large distances horizontally and be ignited by a flame in for instance, a refrigerator.
The pressure inside a propane tank can get close to 300psi so any hose needs to be rated for liquid propane and be properly crimped on to the fittings rather than push on with hose clamps.
Opening the sniffer valve to vent the pressure does release propane into the atmosphere and that can be a huge hazard of there are low places nearby, no wind and ignition sources down low.
Need to shut off the tank valves in the right sequence to avoid disconnecting a hose filled with liquid propane.
I do it in South America when I can't find anywhere to fill the US bottles so have to buy local bottles which are too big to fit into the gas locker. I do it in the open air, well away from any houses or vehicles.
By cooling the bottom bottle with wet cloths and pouring hot water over the top bottle, it is possible to fill the bottom bottle without opening the sniffer valve
