Some above advice is to be taken with great caution. There is no approved DROP IN refrigerant approved by the EPA, including Freeze 12:
"Misleading Use of "Drop-in" to Describe Refrigerants
Many companies use the term "drop-in" to mean that a substitute refrigerant will perform identically to CFC-12, that no modifications need to be made to the system, and that the alternative can be used alone or mixed with CFC-12. However, EPA believes the term confuses and obscures several important regulatory and technical points. First, charging one refrigerant into a system before extracting the old refrigerant is a violation of the SNAP use conditions and is, therefore, illegal. Second, certain components may be required by law, such as hoses and compressor shutoff switches. If these components are not present, they must be installed. See the section below on use conditions for more information on these points. Third, it is impossible to test a refrigerant in the thousands of air conditioning systems in existence to demonstrate identical performance. In addition, system performance is strongly affected by outside temperature, humidity, driving conditions, etc., and it is impossible to ensure equal performance under all of these conditions. Finally, it is very difficult to demonstrate that system components will last as long as they would have if CFC-12 were used. For all of these reasons, EPA does not use the term "drop-in" to describe any alternative refrigerant."
Thats the first thing. Second, if converting to R134a or even Freeze 12, not necessary, although, advisable to install new more efficient condenser IF you can cram it into our spider noses- good luck.
Third, the ONLY drop in that does not react with halogenated freon residues in oil which has not been changed out is Propane/isobutane mix (Duracool, HC Refrigerants, ES Refrigerants- all the same thing. propane will NOT react with those residues to produce corrosive acids. otherwise, forget the magic bullet of "drop in" gas without flushing all oil/freon.