As you may know, R-12 refrigerant is becoming scarce and costly. It is just a matter of time before you will have to convert your systems to R-134a. R-134a retrofits are provided by STC or service bulletin depending on the model of aircraft and could be as simple as replacing the receiver/dryer, a few fittings and the refrigerant. Or, it may require replacing the compressor/condenser assembly as well as the preceding parts. If the compressor/condenser is not changed, the R-134a does not operate as efficiently with the R-12 parts since they operate at slightly different pressures.
It is also worth noting that the two refrigerants use different lubricating oils and, although not compatible from new, in a system that has been charged with R-12 the compressor/condenser operates fine with R-134a since the oil has already been circulated to the bearings. Even after the refrigerant has been changed the oil remains in the bearings, thus not affecting bearing life. However, the seals of the R-12 compressor can be affected due to the higher operating pressure of the R-134a. Depending on the climate the change in efficiency may not be noticeable, but certainly worth considering. If the system is not operating very well or the compressor seems to be weak with R-12 then switching to R-134a is not going to fix that, those parts will need to be replaced before charging the system.
It is my recommendation that if doing the retrofit, replace the compressor/condenser to get the most efficiency possible from the refrigerant and prevent the failure of the seals due to the higher operating pressure.
For more information on this and other Citation technical issues, your Duncan Aviation Citation Tech Reps