So what's the cost of this change, and what's the measurable benefit in terms of bad weather avoided?

Cooling with Less Warming: EPA Starts Refrigerant Changeover for Car Air Conditioners | David Doniger's Blog | Switchboard, from NRDC

The Environmental Protection Agency has issued a pair of decisions that begin a changeover of the refrigerants that do the cooling in car air conditioners. Over the next few years, it will be out with the old (HFC-134a) and in with the new (HFO-1234yf). We’ll be able to cool our cars with less global warming.

Both chemicals are hydrofluorocarbons, but the 134a in your car today is a “super greenhouse gas” with 1,430 times the global warming kick, pound for pound, of carbon dioxide. The new refrigerant, 1234yf, has just 4 times the global warming potency of CO2 and will cut the climate damage from car air conditioning by more than 300-fold. Despite its geeky name, changing over to 1234yf will be a big step forward, because car air conditioning is one of the biggest, leakiest, and fastest growing uses of HFCs worldwide. And if HFC growth is left unchecked, these chemicals will be responsible for a major share of future climate change.