Hydrocarbon (hychill) refrigerant that you are talking about is not unsafe (if you consider people drive around with LPG tanks in the boot) and is not more flammable or toxic than the old refrigerants. It actually has an advantage because aromatics are added to it so you can smell if you have a leak. The problem? Very expensive at the moment due to low demand, I have suspected LPG will most likely work as hydrocarbon refrigerants has a composition very similar to LPG.
I have seen quite a few conversions of R22 and R404 to hydrocarbon without any issues, if you're really tight you could get away without replacing the expansion valve.