Per my HVAC contractor, the refrigerant for the new units will cost much more than the refrigerant for current models.
I would love to see a contractor who can speak to the difference in the coolant. I could be wrong. But I remember what a ripoff the auto air folks pulled when they did R12 'conversions.' The auto air business handed customers who went from R12 to R134a big bills for minimal effort and those who stayed on R12 were charged an arm and a leg, too.