[Jim Sims]

If there is poor performance in the system with R-134a a switch back to R-12 isn't likely to solve the problem(s). Also, a typical R-12 automotive system with traditional non-barrier hose will lose about 12% of its charge per year, a 911 with it's extensive hose will lose even more - using R-12 is not a solution to that problem. That being said, one can only convert back to R-12 without changing the oil if the conversion was done with ester refrigerant oil. If the conversion used conventional PAG oil then the PAG oil will have to be drained and replaced (I would also fully flush the system and replace the receiver/drier). Conventional PAG oil breaks down rather quickly in the presence of chlorides resulting from the interaction of the R-12 and interior metal surfaces of the A/C system. There are "double end capped" PAG oils which will work with R-12 but they are more expensive and the likelihood such an oil was used in past conversion I would judge to be quite small. So unless one knows for sure what the current R-134a refrigerant oil is, conversion back to R-12 is not straight forward.

Yes, systems based on CO2 or even ordinary air as a refrigerants are being considered. The future is coming whether we are ready or not.